Genus condensate trap?

I have a micro genus boiler. When I have heating on and then run hot water the boiler soon shuts down giving a77 fault. On examination the condensate trap in full. I empty the trap but it will fill again in a few hours when the hot water is run. It appears to fill in one rush of water. I have taken the trap out and the outlet pipe is clear and runs downhill.

What is causing the trap to fill so quickly?

Is there a filter that I need to clean or will I have to call someone out?