Oke first your test
C:\test exiftool -author=me test.mie
1 image files created

C:\test exiftool -author=you test.mie
Error renaming test.mie

Then i found your the piece in the exiftool code that does the renaming and copied it into a simple test
This WORKS....... its is indeed very strange...
I tried to copy it into this forum but it doesnt want to except it