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.
thanks again, your previous advice resolved the issue, I updated Nodebb which reverted to the previous version of WE (0.2.18), I'm going to leave it as it is, really don't like messing around with the back end stuff, I'm still a noob.