There are several common reasons a protein skimmer will overflow and/or create excess micro bubbles but the most common is a chemical within the water itself.
Chemical Reaction - Some chemicals react with the air that the skimmer produces and causes the skimmer to over produce foamate. Chemicals known to cause skimmers to overflow and/or produce excess micro bubbles are, red slime remover, medications, reef/frag putties (epoxy Putty) frag glues, water conditioners, dechlorinators, stress coat, NEW filter socks, newly mixed salt, bacteria (seeding) additives as well as other dosing medias. In order to remove these chemicals from the water column we suggest that you do several 20% water changes over the period of a month and add a good amount of fresh activated carbon to the system and change out every week till the skimmer has returned to normal operation.