All noise reduction was turned off and I even managed to get one 10sec exposure, before the camera went into tilt mode (and does ever since, if I try to expose for anything longer than 0,7sec @ iso H2).
No lens caps attached.
I thought of the noise too, but the first try went without _any_ problems, after that I shot some more frames at different ISOs (6400-51200) - no problems either.
Only when I went back to ISO 102400 the camera refused to expose properly. And I just tested it one more time: exposures up to 0,7sec no problem at all, 1sec -> tilt.
I'll try to post the image I managed to get out of it later this day.
Thanks for the serious replies!