maxbuttons pro not exporting correctly

I have maxbuttons pro on two websites (running Weaver xtreme).
One one of them the exported butons will not import (not even back to itself), although it does import buttons from the other site. When I try to import i get a ‘filename: button pack could not be unzipped’ message. On the other site which is very similar, they import and export with no problem. Can anyone think of how i can correct this as I dont particularly want to rebuild every button (there are about 40)
thanks!
regards
Mike

https://www.ads-software.com/plugins/maxbuttons/