Comment 15 for bug 354272

Don't move anything to that -bak directory again. It's there as a backup for your original configuration.

Let's first destroy anything pidgin:
sudo apt-get remove --purge pidgin pidgin-data pidgin-facebookchat pidgin-musictracker pidgin-otr libpurple0
rm -R ~/.purple

This should clean out any junk on your system. Next, install JUST pidgin and it's dependencies
sudo apt-get install pidgin

Then try running things again.

If this doesn't work, then I think it's time to find an upstream developer that knows a little bit more about what's going on. I'm definitely getting to the end of my knowledge here...