Hi Andreas,
This behavior is extremely strange. We haven't seen it before, and of course we are unable to reproduce it on any platform and machine. Keep in mind that the Linux version is the reference implementation of PixInsight, so it is the most stable, reliable and "perfect" version of PixInsight by far. A problem of this kind cannot pass unnoticed, of course.
Linux Mint is not one of the Linux distributions we use for testing (they are, by order of importance, Red Hat Enterprise Linux, Fedora and Ubuntu). However, many PixInsight users work on Mint without problems. There is something wrong or strange in your desktop or video configuration. What hardware are you using? Which desktop environment?