XF 1.4 " Please enter at least one valid recipient." After Login

When I login from my home page, occasionally I will get redirected to a page that says:

- Forums - Error
Please enter at least one valid recipient.

Instead of being taken to the page I was on or the home page. It doesn't happen every time. It is the message I get when trying to send a PM to an invalid user. The URL on this page is:


And when I try to view the source Firefox says it no longer exists, which I have never seen before. Nothing shows up in my server error logs. Any ideas?
It does happen with an all default style, but it does not happen when add-ons are disabled.

Where would be a good place to try to find what add-on maybe hijacking the redirection process? If I could just get a general pointer on where to look I think I can dig it out myself.
Figured out the issue:

Alter Ego Detector by Liam W. I can't have it send a PM when it detects a duplicate account to the same name it is sending the alert from (from my admin account to my admin account). Fixed this and the issue resolved itself.