Hi Joel,
I think you've almost answered your own question 
judging from the error message, usernames are made lower case at some point and Crowd is probably case sensitive. I don't recall if this is actually the case, but perhaps you can try changing a user from upper to lower case?
The fact that the user can login on the various atlassian products is probably an indication of either the API openfire is using, or openfire itself, is case sensitive.
Unfortunately i no longer have access to a setup with openfire and crowd, so i can't really help troubleshoot the actual plugin.
Cheers,
Dennis