Are they doing the same as what I just did and attempting to login from your forum pages that only accept a username, where as your main site login page accepts an email address or username?
For help with this you will need to get the users to explain step by step each thing they do when the error occurs so it can be replicated.
If they are doing things correctly and it randomly doesn’t work you should also be experiencing the problems.
I’ve just been testing this on my sites and I can’t get it to fail. Are there any more details in the server error log, do you have any custom code or bbpress plugins installed?
Also does debugging show any additional info?
