It is possible that this is a false alarm.
I wrote an
article on my Polish blog, rebooted the device just to be sure. And it turns out that the MOBI books that did not open are now working again! Did Amazon read the article and remotely fix the software? Actually, no.
I opened ten files, and on the eleventh it crashed again. I went back to the tenth and .... would not open either. After another reboot, it again seemed that some files were opened and some were not. Maybe there is a lack of memory that is not cleared after previous files?
It may also be an individual problem with my device. I'm going to replace it because of the yellowish stripe, so I'll check after I replace it.