A bit more detail after some investigation:
The modality tag "SEG" is ambiguous. It is used for the SOPClassUIDs SurfaceSegmentationStorage (1.2.840.10008.5.1.4.22.214.171.124) AND SegmentationStorage (1.2.840.10008.5.1.4.126.96.36.199).
The reader is only intended for the latter one. I attached one test file for each case.
SurfaceSegmentationStorage triggers an exception. This can be avoided by directly checking the SOPClassUID in the mimetype method MitkDICOMQIMimeType::AppliesTo.
SegmentationStorage triggers a crash. Needs further investigation.