It was threadadmin_authenticate that was causing the problem I think.
Although the template wasn't changed at all (wasn't red), I copied everything in there, and reverted it anyway.
Upon reverting it the button still wasn't fixed and now my header is out of sync. I copied the original code back in, the button still doesn't work and the header is still out of sync!
Total nightmare!

Is this worth all the hassle or will i simply remake my template to work on 4.0.5?