While I struggled with this also on my 2215 classic I finally realized what it was. I have a 18" airstone against the back of the tank and at first didn't think it was close to the intake because that's the first place I looked.
So I reprimed the filter, rechecked all the hoses, followed eveyone's helpful advice including Eheim's support. Nothing worked, so I decided just to turn off all my air and observe, and low and behold it stopped purging with the micro bubbles. So I restarted the air, sure enough every so often a cycle of bubbles would purge out the return.
I decided to watch the intake very closely for approx 4-5 minutes and observed what was happening. A very small stream of bubbles from the airstone were close enough to have some sucked in every so often, this led
to the build up of air and the subsequent purging. By moving the airstone over approx 3-4 inches further from the intake this has caused the mocro bubbles in the filter to disappear completely.