@vladcr - there have been a couple of people over the last few months who have posted here about that issue. Having not experienced it myself nor received enough detail from those who got it fixed I cannot say the exact cause, other than it does not appear to be related to a specific plugin. It sounds like some sort of corruption of your Calibre install.
Note that uninstalling and reinstalling Calibre by itself is not the same as a complete reinstall, because your local Calibre configuration directory containing your configuration files and plugins is not touched. This is a good thing when you are upgrading of course, but if the problem lies in one of those folders it won't disappear by just reinstalling Calibre itself.
If I can get a more definitive answer as to the cause I will put something in the sticky thread. Right now all I can suggest is the following:
- Read the posts related to what the previous users have reported and tried in this thread. Loeffel did at least get their system back up and running again so you could work your way towards what they said worked in the last post though perhaps not quite as extreme as installing an earlier version. I don't believe the discussion in the other link I put in that thread pointing to the posts in Kindle Collections was closed out by the user reporting back.
- Have you installed any other software on your PC recently? Perhaps there is some sort of conflict, Python related in particular. Try uninstalling that software to see if that makes a difference.
- If you try the "complete clearout" route, make a copy of your Calibre configuration directory before deleting it (i.e. just rename it). I mean the folder that opens when you go to Preferences->Miscellaneous->Open calibre configuration directory. Then try starting Calibre without reinstalling it again and see if installing the plugin now works. If it does, at least that would tell us the problem lies in a corruption of your configuration directory, in which case try a comparison of them yourself or if unsure zip them up and post a link or PM me with it.
I would like to understand why it occurs to have an answer for any others, so if you are able to take a more diagnostic approach to solving it that would be great.