From time to time, entire blocks from different pages disappear. I can’t trace the trigger for this case. This is very important and dangerous because sometimes an entire page of blocks disappears - several hours of work.
Can you give me some advice, where can I read about such cases and how can they be avoided?
Welcome. Do you use sync? If so, have you checked the FAQ? e.g.:
Hello. I use dropbox sync.
Sync is well known of being problematic, including losing work, unless ensuring that:
When failing the above and losing work, try Dropbox recover.
Also in addition to what @mentaloid said, be weary of sync services running when you’re working in Logseq.
They will attempt to sync your files continuously and can cause issues and conflicts/data loss that way.
Whenever I forget to turn off my sync service Logseq will throw errors as the sync service locks files for syncing while Logseq is trying to write.
This is because Logseq will very frequently write to file, triggering the sync service to sync the file.
Do you not sync your notes at all or does it only happen once a day or something?
I turn sync back on when I’m done with Logseq. Though for that sync service it is only used as a backup on my PC, so I only have that issue if I happen to work on my PC. Which is not very often anyway. To sync across devices I use Git, because I like keeping things in my control 
Hi. Can you tell me where I can find instructions on how to set up github synchronization?))))
It depends on your devices. You’ll need a place to store your files as well such as GitHub or similar.
It’s not easy and there aren’t really straightforward instructions. I’ve searched around and cobbled it all together piece by piece myself.
It does require some knowledge obviously.
So searching on this forum and googling will be your best bet.
Even if you don’t sync via git, I think using git for versioning can be useful in guarding against data loss. Just Google how to start a git repository. It does not need to be uploaded to a git server. Once you have your folder set up as a git repository, you can turn on enable git auto commit under version control in settings. Now you can use view page history in Logseq the next time things go missing. Not an ideal solution, of course. I think syncing with git is great if you can make it work because you can see the changes you’ve made and decide how conflicts are merged manually. Git on Android is fairly simple (just install Termux and learn some git commands), but it was kind of a pain on iOS when I used an iPad.
Can’t agree more with this sentiment
I’m still frustrated with my iPad half the time lol.