I dont use htaccess or mod rewrite in the code so it would have to be something relative to your server it would appear and would make sense since your getting a server error.
There is no code difference whether its a new bid or another bid.
So you need more info from your host on what leads them to that conclusion. try removing any htaccess files you have on your server since thats what they are saying it is. I have already tested the default code so feel free to see what I mean
test bidding - PhotoPost Classifieds