According to Windows Task Manager, there are 30 threads running on this process. But my GUI is simply running on main thread. There should be nothing graphical going on in any of my background threads.

Here's an interesting part: I said previously that ST/MT in Painter did not make any difference. Well, it did not when I had started the program originally with Painter MT enabled and then switched 'live' to ST to test for the error effect. BUT: Now I started with Painter fixed in ST and: Error is gone... Sounds like in my original MT/ST test the font caching in Painter had already happened in MT before I switched it to ST.

I need to dig deeper on this.

BR, Tom