Ok, I did what kovidgoyal asked and, as described in my very first post in this thread, this is when the bug pops up:
1. start calibre (virgin if you must)
2. enable automimport, set it to a folder of you choice
3. restart calibre, autoimport works
4. go to preferences|add books and enable some conversions (here epub, mobi)
5. restart calibre, autoimport stops working
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