That's an sff fault. Normally caused by a DOS character in winmugen. Was the one on your mac an old dos version you ran through a specific method on the mac itself. (Or the linux one of course) both of these used slightly different SFF standards to winmugen and they would often need to be recompiled.
Your problem here is more establishing if this is the case. Open your select def, comment out every character. See if the one you have left works, if no, you have problems, if yes uncomment half of them and load up each and every one of them. Use simul vs to speed things up a little. If all those work, uncomment the other half and do the same thing. If you find a set of 4 that causes an issue, load each one in it's own training mode vs itself until you pin down the problem child, there may be more than one.
The only reason i'm suggesting the comments is because mugen will pre-cache characters and it may wind up loading one that you're not actually testing on.
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