Just registered to confirm, We have the same issues in our company with the HP LaserJet Pro 400 MFP M425dn.
Send fax is working normaly on a computer running Windows Xp, Windows 7 64 bit (updates off), but on the Windows 8.1 update 1 I get a Fatal Error Unable to communicate with the device as mentioned above.
thanks, ClaxtonHouse for making a stand (as seen in the other topic)
Hp really needs to sort this out, its a needed feature of the mentioned MFPs.