Can't create issues in Opera

Since upgrading to 3.4.1 users have been unable to create an issue with Opera.

In Opera, a user creates an issue, and assigns it to someone (either themselves, or anybody from the dropdown box).
However, upon clicking 'create', they then see the error 'Issues must be assigned' (see screenshot).

If they switch to IE on the same computer this works. This was previously working in both Opera and IE, in 3.3.2