Kitchen sink (double) with gargbage dispposal, also input from dishwasher.Vent to this network is an under-sink in-line vent. Everything flowing well until neighbors helping with dishes last night; suddenly sink will not drain (either side).
Disassembled trap and drains all clear. Opened clean-out in basement and water drains normal. No blockage in rest of drain line from clean-out to sewer main line. As soon as cap is put on the clean out the water backs up and does not flow. I think it must be something with the under sink vent.Can it clog or stick?

Sure sounds like a vent issue to me. Replace the AAV and see if that fixes the problem. Good luck, Tom