The action requested was not allowed

[eluser]Kola[/eluser]
Okay, so I read http://ellislab.com/forums/viewthread/163976/ and apparently it's an issue with the CRFS token. However, mine is a bit more weird. I have a website where I can log in. I have tried logging in on 6 different computers including many different browsers and OS'. However, the people I have made the website for has had issues with this error. They cannot log in, and have also tried numerous computers, different browsers, a tablet and a computer at one of their work places. I cannot recreate this error, and I am completely oblivious to what can provoke it.

Any ideas or links regarding this would be awesome.