To troubleshoot it, remove the filter and tape the metal latch that stops the bag door being fitted without it up out of the way. Remove the bag, rip off the plastic collar and fit that back into the machine so the bag door closes. Turn it on, does it still do it?
You've removed all possible 'obstructions' from the airflow path, so if it still happens it's either blocked still or faulty in some way.
You checked the cleanerhead, from the brushroll, through it's suction path, split the machine into it's 2 parts and checked all in there? A nice long length of flex to help rod it through will help, I've had an x4 in which did exactly what yours is doing and had a small childs toy wedged in the cleanerhead just past the blockage check flap!