Hello Hugo,
many tx for the quick response. I tried your suggestion, but it didn't work
After analyzing PHP code the whole day together with a colleague, it turned out, that there are some issues in the application code, which may cause this problem. My colleague already addressed it to the developers and we'll see, what comes back.
So far, I can only say (again) many tx for your help and it's (again) no Hiawatha issue.
Cheers
Kai