well... thanks for checking nevertheless!
IM has a great deal of great options - especially the relationship-system is super perfect for me! - and I tend to try them all...
I also tried the DB-portability options with automatic relocation, and setting username alias, ...
So maybe I have found a not yet tested and unforseen combination of settings that is giving such problems.
I rebuilt the DB now from scratch, and currently it works again... like a charm!

Cheers Werner
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P.S. still would love to find the problem, to avoid running into it again... so if there are any additional debugging options, ... I think I could find the time to try them too

e.g. I could also upload the - not working and almost empty db with just a few dummy pics inside ?