taken from IslandConnections:
The glitch affected the Spanish government's SARA data base, introduced in November 2014, which allows airlines and ferry operators to automatically check that passengers living in the islands, including EU nationals, are registered with their local town hall and therefore eligible for the substantial discount. Many passengers who turned up on Friday evening to buy tickets at airports and ports for immediate travel were told they would have to produce a certificate from their Ayuntamiento - an impossible mission as town halls had already closed - or pay the difference between the subsidised fare and the full fare for their journey. A number opted to cancel their plans to sail to La Gomera from Tenerife for the long Bank Holiday week-end due to the unanticipated extra cost. No explanation has been given for the computer failure by the Ministry of Transport.
(a taste of what it is like to be an Extranjero - we know not to move without our travel empadrons! ;D)