Database onMigrate RuntimeException

From gplay console reported for 21.2.0 RC1

Exception java.lang.RuntimeException:
  at androidx.room.BaseRoomConnectionManager.onMigrate (RoomConnectionManager.kt:212)
  at androidx.room.RoomConnectionManager$SupportOpenHelperCallback.onUpgrade (RoomConnectionManager.android.kt:152)
  at net.zetetic.database.sqlcipher.SupportHelper$1.onUpgrade (SupportHelper.java:27)
  at net.zetetic.database.sqlcipher.SQLiteOpenHelper.getDatabaseLocked (SQLiteOpenHelper.java:403)
  at net.zetetic.database.sqlcipher.SQLiteOpenHelper.getWritableDatabase (SQLiteOpenHelper.java:278)
  at net.zetetic.database.sqlcipher.SupportHelper.getWritableDatabase (SupportHelper.java:60)
  at androidx.room.driver.SupportSQLiteDriver.open (SupportSQLiteDriver.android.kt:26)
  at androidx.room.driver.SupportSQLiteConnectionPool.getSupportConnection (SupportSQLiteConnectionPool.android.kt:35)
  at androidx.room.driver.SupportSQLiteConnectionPool.useConnection (SupportSQLiteConnectionPool.android.kt:42)
  at androidx.room.RoomConnectionManager.useConnection (RoomConnectionManager.android.kt:126)
  at androidx.room.RoomDatabase.useConnection$room_runtime_release (RoomDatabase.android.kt:593)
  at androidx.room.util.DBUtil__DBUtil_androidKt$performSuspending$$inlined$compatCoroutineExecute$DBUtil__DBUtil_androidKt$1.invokeSuspend (DBUtil.android.kt:113)
  at androidx.room.util.DBUtil__DBUtil_androidKt$performSuspending$$inlined$compatCoroutineExecute$DBUtil__DBUtil_androidKt$1.invoke (Unknown Source:8)
  at androidx.room.util.DBUtil__DBUtil_androidKt$performSuspending$$inlined$compatCoroutineExecute$DBUtil__DBUtil_androidKt$1.invoke (Unknown Source:4)
  at kotlinx.coroutines.intrinsics.UndispatchedKt.startUndispatchedOrReturn (Undispatched.kt:61)
  at kotlinx.coroutines.BuildersKt__Builders_commonKt.withContext (Builders.common.kt:163)
  at kotlinx.coroutines.BuildersKt.withContext (Unknown Source:1)
  at androidx.room.util.DBUtil__DBUtil_androidKt.performSuspending (DBUtil.android.kt:251)
  at androidx.room.util.DBUtil.performSuspending (Unknown Source:1)
  at androidx.room.RxRoom$Companion$createMaybe$1.invokeSuspend (RxRoom.kt:83)
  at kotlin.coroutines.jvm.internal.BaseContinuationImpl.resumeWith (ContinuationImpl.kt:33)
  at kotlinx.coroutines.DispatchedTask.run (DispatchedTask.kt:104)
  at java.util.concurrent.ThreadPoolExecutor.runWorker (ThreadPoolExecutor.java:1145)
  at java.util.concurrent.ThreadPoolExecutor$Worker.run (ThreadPoolExecutor.java:644)
  at java.lang.Thread.run (Thread.java:1012)

There seems to be something wrong with DB Migrations.
According to https://github.com/nextcloud/talk-android/milestone/96?closed=1 the only PRs that touch the migrations are

• https://github.com/nextcloud/talk-android/pull/4971/
• https://github.com/nextcloud/talk-android/pull/4973/

The database was also touched in

• https://github.com/nextcloud/talk-android/pull/5155

Please note:

• This exception will postpone the 21.2.0 release to production at least by 2 weeks as i won't release this before my vacation (4.8.-16.8.).
• This has NOTHING to do with https://github.com/nextcloud/talk-android/pull/5175 which is not included in the stable-21.2 branch.

Start with androidx.room.BaseRoomConnectionManager.onMigrate at RoomConnectionManager.kt:212 and the migration-related changes in PRs 4971 and 4973. Compare those with the database changes in PR 5155, then reproduce the 21.2.0 RC1 upgrade failure. Done means identifying and resolving the migration exception and confirming the database upgrade succeeds.