Help! SD card in device not detected

I was working with Calibre today and everything went fine for many hours. After restarting my PC, Calibre didn't detect the SD card in Pocketbook Touch Lux 2 anymore. I even did a system backup restore and changed my antivirus software. Same problem. The card shows up in Windows explorer and other programs but not in Calibre.

System is Windows 7 64bit with the current version of Calibre.
I checked ignored devices and the device plugin, everything seems to be fine.