[Adoby]

There are a lot of things you can try first, to try to figure out what the problem might be. I suspect that it is your computer that is the problem. Either something that has gone wrong with the operating system, the filesystem, other software or the hardware. If it was something wrong with calibre, then many more would have noticed.

I don't think it will help to delete and rebuild the database. But that is only a guess.

Create a new small test library and add a few books, and see if the problem persists. Use only books that you are sure are OK. For instance download a few books from this site. If it works then you know that the problem is in the old library. You could switch over and simply copy the books from the old library to the new. It will most likely take a while. It may go faster if you temporarily disable antivirus software while calibre is copying books. Worth a try. Could be prudent to disconnect from the Internet while the antivirus software is disabled.

If this doesn't work:

You should run all diagnostic software on your computer to see if there are any problems with the harddrives, memory or operating system. Apply all updates available.

Check free space on the harddrive and remove junk files. Especially clear the temp folder.

Uninstall and reinstall calibre.

Try to run calibre as another user, perhaps even as root/administrator to rule out any problems with rights in the filesystem.

Disconnect from the Internet and temporarily disable any extra security software like antivirus software and so on. Try to run calibre again.

Make a backup of your calibre library, and restore it on another computer and verify that it works OK. Could be just a small test library. If calibre works fine on the other computer then you know for sure that there still is something wrong with your computer. You could try to reset it or reinstall everything from scratch.