An error that I'm unable to understand ("Unexpected difference in fileset")

I report below the backup log sent via mail. I don’t know what to do now to repair the backup archives.

Failed: Si sono verificati uno o più errori.
Details: System.AggregateException: Si sono verificati uno o più errori. ---> System.AggregateException: Unexpected difference in fileset 2, found 12810 entries, but expected 12811 ---> System.Exception: Unexpected difference in fileset 2, found 12810 entries, but expected 12811
   in Duplicati.Library.Main.Database.LocalDatabase.VerifyConsistency(Int64 blocksize, Int64 hashsize, Boolean verifyfilelists, IDbTransaction transaction)
   in Duplicati.Library.Main.Operation.Backup.BackupDatabase.<>c__DisplayClass28_0.<VerifyConsistencyAsync>b__0()
   in Duplicati.Library.Main.Operation.Common.SingleRunner.<>c__DisplayClass6_0.<RunOnMain>b__0()
   in Duplicati.Library.Main.Operation.Common.SingleRunner.<>c__DisplayClass5_0`1.<<DoRunOnMain>b__1>d.MoveNext()
--- Fine traccia dello stack da posizione precedente dove è stata generata l'eccezione ---
   in System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   in System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   in Duplicati.Library.Main.Operation.BackupHandler.<RunAsync>d__18.MoveNext()
   --- Fine della traccia dello stack dell'eccezione interna ---
   in Duplicati.Library.Main.Operation.BackupHandler.<RunAsync>d__18.MoveNext()
   --- Fine della traccia dello stack dell'eccezione interna ---
   in CoCoL.ChannelExtensions.WaitForTaskOrThrow(Task task)
   in Duplicati.Library.Main.Controller.<>c__DisplayClass15_0.<Backup>b__0(BackupResults result)
   in Duplicati.Library.Main.Controller.RunAction[T](T result, String[]& paths, IFilter& filter, Action`1 method)
---> (Eccezione interna 0) System.AggregateException: Unexpected difference in fileset 2, found 12810 entries, but expected 12811 ---> System.Exception: Unexpected difference in fileset 2, found 12810 entries, but expected 12811
   in Duplicati.Library.Main.Database.LocalDatabase.VerifyConsistency(Int64 blocksize, Int64 hashsize, Boolean verifyfilelists, IDbTransaction transaction)
   in Duplicati.Library.Main.Operation.Backup.BackupDatabase.<>c__DisplayClass28_0.<VerifyConsistencyAsync>b__0()
   in Duplicati.Library.Main.Operation.Common.SingleRunner.<>c__DisplayClass6_0.<RunOnMain>b__0()
   in Duplicati.Library.Main.Operation.Common.SingleRunner.<>c__DisplayClass5_0`1.<<DoRunOnMain>b__1>d.MoveNext()
--- Fine traccia dello stack da posizione precedente dove è stata generata l'eccezione ---
   in System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   in System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   in Duplicati.Library.Main.Operation.BackupHandler.<RunAsync>d__18.MoveNext()
   --- Fine della traccia dello stack dell'eccezione interna ---
   in Duplicati.Library.Main.Operation.BackupHandler.<RunAsync>d__18.MoveNext()
---> (Eccezione interna 0) System.Exception: Unexpected difference in fileset 2, found 12810 entries, but expected 12811
   in Duplicati.Library.Main.Database.LocalDatabase.VerifyConsistency(Int64 blocksize, Int64 hashsize, Boolean verifyfilelists, IDbTransaction transaction)
   in Duplicati.Library.Main.Operation.Backup.BackupDatabase.<>c__DisplayClass28_0.<VerifyConsistencyAsync>b__0()
   in Duplicati.Library.Main.Operation.Common.SingleRunner.<>c__DisplayClass6_0.<RunOnMain>b__0()
   in Duplicati.Library.Main.Operation.Common.SingleRunner.<>c__DisplayClass5_0`1.<<DoRunOnMain>b__1>d.MoveNext()
--- Fine traccia dello stack da posizione precedente dove è stata generata l'eccezione ---
   in System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   in System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   in Duplicati.Library.Main.Operation.BackupHandler.<RunAsync>d__18.MoveNext()<---

---> (Eccezione interna 1) System.AggregateException: Si sono verificati uno o più errori. ---> System.Exception: Unable to find log in lookup table, this may be caused by attempting to transport call contexts between AppDomains (eg. with remoting calls)
   in Duplicati.Library.Logging.Log.get_CurrentScope()
   in Duplicati.Library.Logging.Log.WriteMessage(LogMessageType type, String tag, String id, Exception ex, String message, Object[] arguments)
   in Duplicati.Library.Main.Operation.Backup.FileEnumerationProcess.<>c.<Run>b__1_3(String rootpath, String path, Exception ex)
   in Duplicati.Library.Utility.Utility.<EnumerateFileSystemEntries>d__23.MoveNext()
   in System.Linq.Enumerable.<SelectManyIterator>d__16`2.MoveNext()
   in Duplicati.Library.Main.Operation.Backup.FileEnumerationProcess.<>c__DisplayClass1_0.<<Run>b__0>d.MoveNext()
--- Fine traccia dello stack da posizione precedente dove è stata generata l'eccezione ---
   in System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   in System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   in CoCoL.AutomationExtensions.<RunTask>d__10`1.MoveNext()
   --- Fine della traccia dello stack dell'eccezione interna ---
---> (Eccezione interna 0) System.Exception: Unable to find log in lookup table, this may be caused by attempting to transport call contexts between AppDomains (eg. with remoting calls)
   in Duplicati.Library.Logging.Log.get_CurrentScope()
   in Duplicati.Library.Logging.Log.WriteMessage(LogMessageType type, String tag, String id, Exception ex, String message, Object[] arguments)
   in Duplicati.Library.Main.Operation.Backup.FileEnumerationProcess.<>c.<Run>b__1_3(String rootpath, String path, Exception ex)
   in Duplicati.Library.Utility.Utility.<EnumerateFileSystemEntries>d__23.MoveNext()
   in System.Linq.Enumerable.<SelectManyIterator>d__16`2.MoveNext()
   in Duplicati.Library.Main.Operation.Backup.FileEnumerationProcess.<>c__DisplayClass1_0.<<Run>b__0>d.MoveNext()
--- Fine traccia dello stack da posizione precedente dove è stata generata l'eccezione ---
   in System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   in System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   in CoCoL.AutomationExtensions.<RunTask>d__10`1.MoveNext()<---
<---
<---


Log data:
2018-05-04 00:25:34 +02 - [Error-Duplicati.Library.Main.Operation.BackupHandler-FatalError]: Fatal error
System.Exception: Unexpected difference in fileset 2, found 12810 entries, but expected 12811
   in Duplicati.Library.Main.Database.LocalDatabase.VerifyConsistency(Int64 blocksize, Int64 hashsize, Boolean verifyfilelists, IDbTransaction transaction)
   in Duplicati.Library.Main.Operation.Backup.BackupDatabase.<>c__DisplayClass28_0.<VerifyConsistencyAsync>b__0()
   in Duplicati.Library.Main.Operation.Common.SingleRunner.<>c__DisplayClass6_0.<RunOnMain>b__0()
   in Duplicati.Library.Main.Operation.Common.SingleRunner.<>c__DisplayClass5_0`1.<<DoRunOnMain>b__1>d.MoveNext()
--- Fine traccia dello stack da posizione precedente dove è stata generata l'eccezione ---
   in System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   in System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   in Duplicati.Library.Main.Operation.BackupHandler.<RunAsync>d__18.MoveNext()

This error happened during the database check and I believe it means it expect 12,811 results from a SQL call but only got 12,810 - so something isn’t “lining up” right.

I’d start by using the job “Database” menu and selecting the blue “Repair” button. This shouldn’t take too long and MIGHT resolve the issue.

If it doesn’t, then @Pectojin might have some insight.

Do you recall any changes in Duplicati (an update?) or your system (an OS or other program update) that happened between the last time it ran fine and the first time you got the error?