The air supply for the milk froth is clogged.
Clean the central spout. Proceed as follows:
Pull the central spout all the way down. Remove the cover by pulling it forward.
Push the insert up and out from inside the cover. Clean both parts thoroughly. Then slide the insert back into the cover until it clicks into place.
Remove the nozzles from the central spout.
Pull the rubber stoppers out. Thoroughly clean the nozzles and both rubber stoppers. Then reinsert the rubber stoppers in the nozzles.
When you have finished cleaning, place the nozzles back into the central spout and insert the central spout into the appliance. Firmly push the central spout up so that it fits tightly in the appliance. Put the cover on the central spout. Push the central spout up.
You can now use the appliance again as normal.