#250 XML error in Administration

Try this: as operator, go to Administration, then enter an existing
chatter's nickname in Chatter-administration. Click Search. Poof!
XML not well-formed.

Also, if you enter a nonexistent chatter's name, there's no error or
warning.

[Nobody/Anonymous]

Logged In: NO

Strange, works just fine here (PHP4.3.x, MySQL). I did get
an XML error once upon trying to activate an account, but I
disabled account activation as it wasn't very transparent
towards users.

[Nobody/Anonymous]

Logged In: NO

Note:

Getting the same error, but narrowed it down to a "&"
contained in a string in the LAST_REFERRER field in poc_users.

Manual editing of the referrer URL fixed the problem.

I use '%' to list all users.