Hi Lacko,
It is important that the Time Signature event is at the very beginning of the MIDI file, i.e. at the first tick (displayed in MixMaster as "001:01:0000"). If it is further back in the file, it should be moved to the very beginning.
In addition, it could be that there are several Time Signature events in the file. In this case, all but the first one should be deleted.
Best regards,
Chris