@thomasmorus - I'm on the same setup and I'm not seeing any problems.
First, I'd try the usual windows solution--reboot and see if that helps.
If not, please post a story URL that's giving you this problem.
You can get more debug output by running calibre from command line/command prompt as 'calibre-debug -g'. If you can copy/paste that here (or PM me), that will also help.
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