Airlock in Eheim Canister filter

I need help.

I just set up a 20G planted tank (60 cm), including an Eheim 2222 canister
filter, compressed CO2, and a Fluval surface skimmer. I have to put the
Eheim filter on the same bench as the tank because of space problem. The top
of the filter is about 15 cm below the water level of the tank. I notice the
filter rumbles and pumps out about 5-10 cc of gas every 15-30 min. I have no
idea when the gas is from. It can be the CO2 or the air drawn in through the
surface skimmer. I have reduced the inflow from the water surface to reduce
the air sucked in this way. However, this does not seem to reduce the amount
of gas pumps out through the outflow. Today, the filter developed an airlock
with noisy impeller and no outflow, requiring repriming of the filter.

I wonder if anyone has similar experience or can advise me how to overcome
the problem.

Many thanks in advance.