diff --git a/app/src/main/java/pl/szczodrzynski/edziennik/MainActivity.kt b/app/src/main/java/pl/szczodrzynski/edziennik/MainActivity.kt
index 5b6676d0..55beea7d 100644
--- a/app/src/main/java/pl/szczodrzynski/edziennik/MainActivity.kt
+++ b/app/src/main/java/pl/szczodrzynski/edziennik/MainActivity.kt
@@ -36,11 +36,13 @@ import org.greenrobot.eventbus.EventBus
 import org.greenrobot.eventbus.Subscribe
 import org.greenrobot.eventbus.ThreadMode
 import pl.droidsonroids.gif.GifDrawable
+import pl.szczodrzynski.edziennik.data.api.ERROR_API_INVALID_SIGNATURE
 import pl.szczodrzynski.edziennik.data.api.ERROR_VULCAN_API_DEPRECATED
 import pl.szczodrzynski.edziennik.data.api.edziennik.EdziennikTask
 import pl.szczodrzynski.edziennik.data.api.events.*
 import pl.szczodrzynski.edziennik.data.api.models.ApiError
 import pl.szczodrzynski.edziennik.data.api.szkolny.SzkolnyApi
+import pl.szczodrzynski.edziennik.data.api.szkolny.response.RegisterAvailabilityStatus
 import pl.szczodrzynski.edziennik.data.api.szkolny.response.Update
 import pl.szczodrzynski.edziennik.data.db.entity.LoginStore
 import pl.szczodrzynski.edziennik.data.db.entity.Metadata.*
@@ -628,22 +630,40 @@ class MainActivity : AppCompatActivity(), CoroutineScope {
         app.profile.registerName?.let { registerName ->
             var status = app.config.sync.registerAvailability[registerName]
             if (status == null || status.nextCheckAt < currentTimeUnix()) {
-                withContext(Dispatchers.IO) {
-                    val api = SzkolnyApi(app)
-                    api.runCatching(this@MainActivity) {
+                val api = SzkolnyApi(app)
+                val result = withContext(Dispatchers.IO) {
+                    return@withContext api.runCatching({
                         val availability = getRegisterAvailability()
                         app.config.sync.registerAvailability = availability
-                        status = availability[registerName]
+                        availability[registerName]
+                    }, onError = {
+                        if (it.toErrorCode() == ERROR_API_INVALID_SIGNATURE) {
+                            return@withContext false
+                        }
+                        return@withContext it
+                    })
+                }
+
+                when (result) {
+                    false -> {
+                        Toast.makeText(this@MainActivity, R.string.error_no_api_access, Toast.LENGTH_SHORT).show()
+                        return@let
+                    }
+                    is Throwable -> {
+                        errorSnackbar.addError(result.toApiError(SzkolnyApi.TAG)).show()
+                        return
+                    }
+                    is RegisterAvailabilityStatus -> {
+                        status = result
                     }
                 }
             }
 
-            if (status?.available != true
-                    || status?.minVersionCode ?: BuildConfig.VERSION_CODE > BuildConfig.VERSION_CODE) {
+            if (status?.available != true || status.minVersionCode > BuildConfig.VERSION_CODE) {
                 swipeRefreshLayout.isRefreshing = false
                 loadTarget(DRAWER_ITEM_HOME)
                 if (status != null)
-                    RegisterUnavailableDialog(this, status!!)
+                    RegisterUnavailableDialog(this, status)
                 return
             }
         }
diff --git a/app/src/main/java/pl/szczodrzynski/edziennik/data/api/edziennik/EdziennikTask.kt b/app/src/main/java/pl/szczodrzynski/edziennik/data/api/edziennik/EdziennikTask.kt
index 7a578356..e3692c33 100644
--- a/app/src/main/java/pl/szczodrzynski/edziennik/data/api/edziennik/EdziennikTask.kt
+++ b/app/src/main/java/pl/szczodrzynski/edziennik/data/api/edziennik/EdziennikTask.kt
@@ -90,7 +90,7 @@ open class EdziennikTask(override val profileId: Int, val request: Any) : IApiTa
                 return
             }
 
-            profile.registerName?.let { registerName ->
+            profile.registerName?.also { registerName ->
                 var status = app.config.sync.registerAvailability[registerName]
                 if (status == null || status.nextCheckAt < currentTimeUnix()) {
                     val api = SzkolnyApi(app)
@@ -99,7 +99,11 @@ open class EdziennikTask(override val profileId: Int, val request: Any) : IApiTa
                         app.config.sync.registerAvailability = availability
                         status = availability[registerName]
                     }, onError = {
-                        taskCallback.onError(it.toApiError(TAG))
+                        val apiError = it.toApiError(TAG)
+                        if (apiError.errorCode == ERROR_API_INVALID_SIGNATURE) {
+                            return@also
+                        }
+                        taskCallback.onError(apiError)
                         return
                     })
                 }
diff --git a/app/src/main/java/pl/szczodrzynski/edziennik/data/api/szkolny/SzkolnyApi.kt b/app/src/main/java/pl/szczodrzynski/edziennik/data/api/szkolny/SzkolnyApi.kt
index e6d970b8..3a434001 100644
--- a/app/src/main/java/pl/szczodrzynski/edziennik/data/api/szkolny/SzkolnyApi.kt
+++ b/app/src/main/java/pl/szczodrzynski/edziennik/data/api/szkolny/SzkolnyApi.kt
@@ -5,6 +5,7 @@
 package pl.szczodrzynski.edziennik.data.api.szkolny
 
 import android.os.Build
+import android.widget.Toast
 import androidx.appcompat.app.AppCompatActivity
 import com.google.gson.GsonBuilder
 import kotlinx.coroutines.CoroutineScope
@@ -14,6 +15,7 @@ import kotlinx.coroutines.withContext
 import okhttp3.OkHttpClient
 import org.greenrobot.eventbus.EventBus
 import pl.szczodrzynski.edziennik.*
+import pl.szczodrzynski.edziennik.data.api.ERROR_API_INVALID_SIGNATURE
 import pl.szczodrzynski.edziennik.data.api.szkolny.adapter.DateAdapter
 import pl.szczodrzynski.edziennik.data.api.szkolny.adapter.TimeAdapter
 import pl.szczodrzynski.edziennik.data.api.szkolny.interceptor.SignatureInterceptor
@@ -89,11 +91,17 @@ class SzkolnyApi(val app: App) : CoroutineScope {
         }
         catch (e: Exception) {
             withContext(coroutineContext) {
+                val apiError = e.toApiError(TAG)
+                if (apiError.errorCode == ERROR_API_INVALID_SIGNATURE) {
+                    Toast.makeText(activity, R.string.error_no_api_access, Toast.LENGTH_SHORT).show()
+                    return@withContext null
+                }
                 ErrorDetailsDialog(
                     activity,
-                    listOf(e.toApiError(TAG)),
+                    listOf(apiError),
                     R.string.error_occured
                 )
+                null
             }
             null
         }
diff --git a/app/src/main/java/pl/szczodrzynski/edziennik/data/api/task/AppSync.kt b/app/src/main/java/pl/szczodrzynski/edziennik/data/api/task/AppSync.kt
index c10bab3e..15482c53 100644
--- a/app/src/main/java/pl/szczodrzynski/edziennik/data/api/task/AppSync.kt
+++ b/app/src/main/java/pl/szczodrzynski/edziennik/data/api/task/AppSync.kt
@@ -5,10 +5,13 @@
 package pl.szczodrzynski.edziennik.data.api.task
 
 import pl.szczodrzynski.edziennik.App
+import pl.szczodrzynski.edziennik.data.api.ERROR_API_INVALID_SIGNATURE
 import pl.szczodrzynski.edziennik.data.api.szkolny.SzkolnyApi
+import pl.szczodrzynski.edziennik.data.api.szkolny.SzkolnyApiException
 import pl.szczodrzynski.edziennik.data.db.entity.Metadata
 import pl.szczodrzynski.edziennik.data.db.entity.Notification
 import pl.szczodrzynski.edziennik.data.db.entity.Profile
+import pl.szczodrzynski.edziennik.toErrorCode
 import pl.szczodrzynski.edziennik.utils.models.Date
 
 class AppSync(val app: App, val notifications: MutableList<Notification>, val profiles: List<Profile>, val api: SzkolnyApi) {
@@ -27,7 +30,13 @@ class AppSync(val app: App, val notifications: MutableList<Notification>, val pr
      */
     fun run(lastSyncTime: Long, markAsSeen: Boolean = false): Int {
         val blacklistedIds = app.db.eventDao().blacklistedIds
-        val events = api.getEvents(profiles, notifications, blacklistedIds, lastSyncTime)
+        val events = try {
+            api.getEvents(profiles, notifications, blacklistedIds, lastSyncTime)
+        } catch (e: SzkolnyApiException) {
+            if (e.toErrorCode() == ERROR_API_INVALID_SIGNATURE)
+                return 0
+            throw e
+        }
 
         app.config.sync.lastAppSync = System.currentTimeMillis()
 
diff --git a/app/src/main/res/values/strings.xml b/app/src/main/res/values/strings.xml
index 3d24343d..d81e0ff5 100644
--- a/app/src/main/res/values/strings.xml
+++ b/app/src/main/res/values/strings.xml
@@ -1424,4 +1424,5 @@
     <string name="settings_about_version_details_text">Szczegóły wersji</string>
     <string name="settings_about_version_details_subtext">Informacje o kompilacji</string>
     <string name="build_dialog_open_repo">Sprawdź kod</string>
+    <string name="error_no_api_access">Brak dostępu do API</string>
 </resources>