Hi Friedrich,
thanks a lot for your time!
I finally found the reason: the template sets %_SB_BACKUPDIR% to "%_SB_INSTALLDIR%\BACKUP".
In my somewhat complicated script, the .exe path is defined in the registry (as well as the root path for database and help files).
Part of patching the .exe is copying the to-be-patched-file to a folder (%_SB_BACKUPDIR%) -- which does not exists. And here it bombs with decompression error #3

I set the %_SB_BACKUPDIR to the folder where the .exe resides (<exe-folder>\BACKUP); since the .exe is there in all setuptypes, we are sure that we have an existing backup folder, and backups for .exe and all the db and .chm stuff will be backuped there also.

Resolved - and maybe a contribution t the knowledge base.
regards, André