Thanks for your help but I found that the problem was somewhere else. I used MegaSync to backup my ebook library to Mega servers but thanks to my stupidity it replaced the database file every time I reboot. This happened only if the database file was different then the last stored file and renamed the existing file. That caused calibre not to be able o load the database file because it's name was different then the calibre expected.
I'm sorry for wasting your time but thank you a lot for your help.
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