I recommend if you moved the site to a new location, then delete all existing carts and then all users will have to get a new cart. What happens is we serialize the cart info and the redirecturl into the mp_PayPalLog table and this may have happened before you moved the site, then the user completes checkout but the log still has the original url which may not be correct after moving the site.
Basically we pass the paypallog guid as a private data and paypal passes it back with the pdt request, so we lookup the log and then we re-serialize the cart from what we serialized into the log before they went to paypal (this way the cart cannot have changed while they were away, otherwise there might be more things in the cart that the user did not pay for when they were at paypal).
Other than that I would review the mojoportal logs for any clues or related errors.
Hope it helps,