According to the examples in XEP-0077 the server sends within the registration Data Form the senders address. Missing this causes Pidgin to send the registration to the FQDN of the server which brake the registration process if the hostname of the server differs from the xmpp.domain. This was also discussed in the thread Trouble with the Pidgin IM client and OpenFire.
The attached Patch is fixing this bug. I would appreciate if it will be included in the next releases.