It's possible that one of your users has a special character in their name that causes problems when Geeklog uses it in the email address. Recent versions of Geeklog are a bit better in handling special characters in email addresses, but
not quite perfect yet, either.
Sorry, that doesn't really help you, I assume. See if you can find a user name (or full name) that has some special characters and see if it helps to change them.
bye, Dirk