Some additional information. After yesterday's issue on my PC I felt that the error message seems to be related to me running 32b Win10. I therefore tried the whole today on my laptop running 64b and voilá, the program did start and converted my file.

Funny though that during the conversion I checked the load in the task manager and there the program was listed as a 32b program... 🤔. So not really sure now what the original problem was.