No GUI after reboot - only SSH

That’s correct. I can connect to it only via SSH. The application on my mobile phone cannot find the volumio device and volumio no longer appears in browser on my laptop.

So it looks like a corrupted system.

But 5 times per 2 months?

as @dvo pointed out, a faulty or underamped popwer supply may be causing the problem.
or a micro sd card that is going bad.