Ok, the problem was solved. But I'm not sure how exactly.
After I wrote this post I noticed that the latest version of NodeBB is 1.16.x. But while upgrading the forum i switched to 1.15.x branch (I thought that was the latest).
After that, I used ./nodebb upgrade command. Maybe this was the issue? Maybe, I switched to the previous version branch, but installed the latest version of files with the "upgrade" command? — I don't really know. But when I switched to 1.16.x and upgraded the forum again, then it worked as expected. The forum now works fine.
