If a theme name of a .xml has special chars in it such like é, è, &, etc. the theme import failes because it can't read the contents as it stops just at the 2nd line (Where the theme name is).
If you remove those chars the import will work but it's still a bug so I've reported it.
Example xml file? I don't see how this is a bug with MyBB because we use a 3rd party XML class to parse along with the PHP XML extension and we don't just "stop" on multibyte characters.
Okay I've just added the original default xml with the following name:
TéSt & Tèst
Try to import it and you will get an error.
Thank you for your bug report.
This bug has been fixed in our internal code repository. Please note that the problem will not be fixed here until these forums are updated.
This wasn't caused by the multi-byte characters, it was caused by the & you had in there - It wasn't converted to & and caused the xml document to not validate.