HCL released FP1 for Domino and Notes 12.0.1 today. I wondered if 12.0.1FP1 will contain the code of 11.0.1FP5 that caused a big problem under certain conditions.
Fortunately it doesn't.
In parallel I received a test fix for my issue in 11.0.1FP5 from HCL support which I reviewed but did not test it until now. The Code contains similar code as in 12.0.1FP1 and a marker with the SPR# ASHECCY97E so I am quite confident that this also will solve the problem on 11.0.1FP5 of the customer's machine.
Thanks HCL for the quick fix and also for additionally adding it to the latest FP1 for version 12!