There are several common reasons a protein skimmer will overflow
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 are, red slime remover, medications, reef/frag putties (epoxy Putty) frag glues, water conditioners, dechlorinators, stress coat, fresh 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.
Water level in sump - It is important that you place your skimmer in the proper water level. It is also important that the output of the skimmer is not submerged under water unless it is properly vented.
Inner water level - If your inner water level is set too high the slightest change in your water chemistry can cause your skimmer to over flow. We suggest that your inner water level to be set at no higher than the collar where the cup meets the body.
Blockage - The air intake otherwise known as the venturi, is like small vacuum pulling air from the surrounding environment. Often collecting dust and debris from the air which can block the proper amount of air to enter the pump increasing the water being pulled by the pump. It is also common for saltcreap to build up in the venturi and will need to be removed manually by scraping it away or dissolving in a solution of vinegar and water. Make sure you inspect the air venture, silencer and air hose for any debris or salt creep deposits. Also be sure the air hose is not bent or too long preventing proper air flow into the pump.