Appliance Repair Questions? Ask a Technician for Answers ASAP
The most likely cause is a build up of ice behind the rear panel, I would suggest you perform a 24 hour defrost, secondly try feeding some solder or flexible wire down the condensate tube to check its not blocked.
If this continue to be an issue the next stage is to strip down the appliance, but this would be best done by an engineer since you can damage the cooling elements and fascia when getting to the rear panel.
Let me know if you would like me to recommend a local engineer after you have defrost the appliance and check the tube is clear.