Ok, back to basics questions
What paint /cleaner were you using prior to the problem? Some combinations don’t play well and can cause small particles of paint to become cement and require the right liquids to dissolve them.
Bubbling in the cup like that normally is associated with airflow problems, something isn’t sealing like it should or the air passages are blocked.
you mentioned you screwed it up as tight as f ***, well as
@twood mentioned, that isn’t great for precise equipment!
If you have done a full tear down clean are you sure you haven’t misplaced a washer (easy done but not sure if sotar have a washer at the nozzle end) or cross threaded the placement ring, have you inadvertent flared the nozzle.