Thanks both of you for your answer. unfortunalety I had absolutely no access to the internal memory, all programs including dd and gparted would just hang while trying to access /dev/mmcblk0.

Since I'm going on a trip today and need my N810 for that I tried the radical approach and did a complete reflash (Had not to old backups in place). For some reason this helped and my internal memory is back to normal, including all data that was on there is still there and seem to be ok. So for now the issue is solved, although I don't know what caused the problem and why flashing solved it.