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 lid by pulling it forwards.
Push the insert up and out from inside the lid. Clean both parts thoroughly. Then slide the insert back into the lid until it clicks into place.
Remove the nozzles from the central spout.
Pull out the rubber stoppers. Thoroughly clean the nozzles and the two rubber stoppers. Then reinsert the rubber stoppers in the nozzles.
When the cleaning process is finished, 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 lid on the central spout. Push the central spout up.
You can now use the appliance again as usual.