Second attempt:
I deleted the database and started Duplicati again. It ended with following error:
2018-06-27 23:59:54 +02 - [Warning-Duplicati.Library.Main.Operation.FilelistProcessor-ExtraUnknownFile]: Extra unknown file: duplicati-ifbb1ef804a184fa396e7dcbd16782ffe.dindex.zip
2018-06-27 23:59:54 +02 - [Warning-Duplicati.Library.Main.Operation.FilelistProcessor-ExtraUnknownFile]: Extra unknown file: duplicati-iffea6c4773fa4d7685501f3b61cfb508.dindex.zip
2018-06-27 23:59:54 +02 - [Error-Duplicati.Library.Main.Operation.FilelistProcessor-ExtraRemoteFiles]: Found 418 remote files that are not recorded in local storage, please run repair
2018-06-27 23:59:54 +02 - [Error-Duplicati.Library.Main.Operation.BackupHandler-FatalError]: Fatal error
Duplicati.Library.Interface.UserInformationException: Found 418 remote files that are not recorded in local storage, please run repair
bei Duplicati.Library.Main.Operation.FilelistProcessor.VerifyRemoteList(BackendManager backend, Options options, LocalDatabase database, IBackendWriter log, String protectedfile)
bei Duplicati.Library.Main.Operation.BackupHandler.PreBackupVerify(BackendManager backend, String protectedfile)
bei Duplicati.Library.Main.Operation.BackupHandler.<RunAsync>d__19.MoveNext()