You are most likely suffering the
fix from mIRC 7.34, try using /!join instead of /join, if it works, you are wrongly overriding the /join command with an alias.
You should ask about your .bak file problem in a different thread... but if the content of the file is the same as what you had before (as in, a well structured INI files), renaming them as INI files will work, you're having another, different issue if that does not work for you.