Update:

So far my customer could not provide the QEP and currently I cannot logon to the system (holidays!).

But they ran the Index Rebuild on that table which seems to help a lot! So, for now we will schedule this maintenance daily ...

But this also means, that - for now - we cannot reproduce the problem anymore, so I owe you the QEP ...

Probably we should close this thread - for now. Once the problem returns I'll make sure to get the actual QEP and come back on this again ...

Thanks all for your support!

Happy Holidays,

Jörg