Yes mathmap was not update from long, nevertheless still works with gimp 2.8
So should be something wrong in your installation (maybe some missed or misplaced file ?)
or more simply something wrong happened during download (it is no so rare ...) and some files went corrupted
My suggestion is download again and reinstall .
It is even possible that MM needs some DLL that were in older gimp version but not in 2.8
i can't remember well because i installed long ago but i have a vague memory of a "png" related dll that i had to recover from older gimp)
Anyway if was that you would see a clear gimp error message about " .dll not found" and that will be easy to fix