Hello Kai,
SUMA looks for the alignment transform in the header of the transformed volume. Unfortunately, at the moment neither 3drotate nor the Nudge plugin leave a record of the transformation in the header of the output dataset (unlike 3dvolreg and 3dTagalign). I will look into this later today or tomorrow morning so you'll have a fix soon.
I don't know why 3dvolreg failed, I'll need some data to look into the problem.
Please send me the necessary data (and one anatomically correct surface) to reproduce the error. Also send me the 3drotate command that you used to align the volumes. This way I can test the changes I will make to 3drotate.
cheers,
-ziad