what you can see is my test board using 4.1.10. But there a lot of other modifications installed for testing. maybe there are some incompatibilities

I don't know, sorry
Give me a minute, I will switch off all other Add-Ons except yours, and take a look, whether the issue still appears.
EDIT: I switched off all modifications concerning SHOWTHREAD. The issue is definitely caused by your mod

But I'm sure you will find the error in your code.