might be that you have a
www.yourforum.com versus yourforum.com issue.
so one or the other isn't being allowed, if you have a referrer check.
there are a number of ways to fix this.
1. disable referrer check -- little more open then some would like
2. add both www.yourforum.com|yourforum.com to the valid referrers
3. look over @ ubbdev where blaaskaak posted what he did (also giz added a solution in that thread too)
good luck
oh and ps: i officially request that all 'blaaskaak' are active text --> Yarp™
(its easier to type Yarp! )