Hello,
Tried to follow your recommendations and here is what happened.
I searched for factory restore option, however, seems that it is not available. So I re-formatted by microSDXC cards in FAT32 with 4K clusters. At the same time I had reset media library. I copied all my files back again to cards, inserted them into my FIIO X5 gen 2 and created new media library. Then I started to test my player. First I was playing music at home using IEMs. Player would play 1 hour or up to 1.5 hours without any problem. However, after that the issue reappears again - playing stops, player unlocks itself and screen lights up. After that it is difficult to start playing again. It looked like this might be related to overheating and after 30 min pause playing resumed without any problem - but for limited time only.
And afterwards I tried to use it during a hike with Audeze Sine. Here is where it became really ugly. First interruption happened already after 5 min or so and after that player became completely useless it was stopping every few seconds or so. I had stopped it and switched to listening my phone for awhile. Tried player an hour later - same story, it simply refuses to work normally. Almost looks like it worked somewhat better with cards formatted directly in player with default cluster size (32K). Not sure as well if overheating can be blamed, as I do not expect major overheating after 5 min of playing time.
I might want to re-format cards in player again, however, given the fact that copying music back to cards takes several hours, I'll have to find some free time to do that.