BuddyPress.org

When someone tries to compose a private message, the auto complete function looks likes it is working, although no results are shown. The loading graphic of ajax is running, but it doesn’t find any usernames or display names. Maybe it is due to the fact that I have some plugins running like BuddyPress Real Names and in the past had the plugin BP Display Name running. I have already consulted a lot of online auto complete issues and troubleshooting pages, but none of the given solutions solved my problem. I guess there is a misconfiguration in my database, but I have no idea where to search. I hope someone can help me out with this or give me a clue were to look for. Thanks in advance.
Best regards,

Olaf