Try to specify illegal filename or nonexistent directory

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alter database add difference file '****';

alter database begin backup;

Statement failed, SQLCODE = -902
[335544344] I/O error during "CreateFile (create)" operation for file "****"
[335544733] -Error while trying to create file
[14696098] -(Incorrect file name).

alter database end backup;

Statement failed, SQLCODE = -607
[335544351] unsuccessful metadata update
[335544382] -Database is not in the physical backup mode

alter database drop difference file;

Statement failed, SQLCODE = -607
[335544351] unsuccessful metadata update
[335544832] -Cannot change difference file name while database is in backup mode

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The engine is a bit confused whether it is in backup mode or not :)
The "drop difference file" will succeed only after disconnecting all connections.