Getting "Sorry, the address entered is not valid"

[Grenoble]

Happened few times this week. After posting a reply to a thread, it comes up with:

Sorry, the address entered is not valid
Link to: Proceed to home page

It is posting my reply correctly. Can't see any consistent reason. Posting from a Mac in Firefox.

Most recent thread it happened in was:
http://www.pistonheads.co.uk/gassing/topic.asp?h=0...

Only thing that leaps out is the @ in the thread title - if that is causing an issue?

[Grenoble]

Still getting it, the same thread.

3 or 4 times today.

Is it the @ in the thread title?

[Switch]

It's entirely possible....
I'll try and reproduce it..