Some additional potentially helpful information
… after some days of observation and investigation
After a few (automatic) boot attempts, the phone was “optimizing” apps. It seems that sometimes, it even crashed there (unless this action might actually involve reboots). It did this optimization every now and then after a few crashes (I didn’t count the crashes between these optimizations).
Three days after the problem’s first occurence, the phone did not get past the boot screen for a few days, no matter what I did. It didn’t even get into the charging screen (which it usually does when plugging it in while it’s off).
But some days later, I could charge it over night and the next morning, it was seemingly working again. So I inserted my SIM cards, configured some settings, but installed only some very basic apps (Lightning browser and Telegram messenger).
I recognized though, that since then on each boot up, the phrase “starting apps” pops up for a split seconds. I have either never noticed this before or it just didn’t appear before.
The phone worked seemlessly for two or three days. At some point, out of nothing, the phone asked me for my second SIM card’s PIN. And while I was typing the PIN, it froze and roughly 6 seconds later, it crashed.
Again, I found the phone in the same situation as before … and again, some two days later, it booted and worked again.
I cannot find a pattern yet, but I’m trying to find a connection between the crashed and either the usage time or the date/time, in case it might be a software issue. Latter is supported by the fact that the display sleep timeout is limited to 30 seconds.