[support] access denied when a user register to my web site

[Simone Dal Maso]

Hi,
I'm going crazy!
This is really a serious problem.
When a user attempts to register to my website, it receives the validation 
email, but when he attempts to click to the confirmation link, it receives a 
access denied page.
what can be happened?
ah, I recently moved my website to another hosting, but I don't know the 
cause of this...
help please.
thanks.