Yes, the only plugin I have installed is called "Find Duplicates".
I have deleted Calibre and then reinstalled it, but the problem persists.
I'm adding a screenshot to show how it looks after I open Calibre. As you can see, the "Jobs" windows opens automatically and is in the background.
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To comply more with the forum guidelins I replaced the *huge* image with a half-sized version and added it as an attachment.