Photos only appear in Lightroom after restarting the application

Hello,
I have been an active user of Lr for many years and have never had any major problems with this application. Unfortunately, I have been having a problem for a few weeks now that I can neither deal with nor find any solution for on the internet.

Generally speaking - the problem is that after various operations (importing, exporting, moving files, tagging/flagging files, etc.) the effect of my work can only be seen after exiting and restarting Lightroom.

Examples:
1. I import files from my camera (I have done this hundreds of times) into a folder in the library. I see a progress bar, but no new folder appears in the library, nor do the new photos I'm importing. When the import is complete (when the 'build previews' option does not appear), I have to exit and restart the app and then I see a new folder with the newly imported photos.

2. When I edit the RAW files and export them to jpg files, the files do not appear in the folder. Of course I have the "Add to this catalog" option enabled. Once the export is complete, I have to exit and restart the app and then I see these new jpg files in the folder.

3. Sometimes, but not always, when I tag the files (flag, rating, colour label) nothing appears on the thumbnails on filmstrip. I have to exit the application and restart it to see that the files are properly flagged or rated.

Basically - most of the operations that I do in Lr require a restart of the app so that I can see the effect of these "actions".

Again, I'd like to add that I'm a long-time Lr user and I don't make "schoolboy mistakes" anymore. I know Lr, I know how the application works, I have thousands of photos in my library and this has never happened to me before. I'd also like to add that I did a reinstall, reset preferences and went back to an earlier version of the app and nothing helped. 

I am currently using the latest version of Lr.