After checking that you can open port 53 udp yourself with, say, nc (which you tried), strace the binary that tries to open port 53 and fails, and find the system call that fails. You can compare it with an strace on nc to see how it differs.

If this doesn’t clue you in (e.g., you see two attempts to listen to the same port…) Next step would be to find in the source code where it fails (look for the error message printout) and start adding diagnostic printouts before the failing system call and compile and run your edited version.

Especially if the media is readily available elsewhere which is always the case for movies you “bought” digitally.

Except when they aren’t. Especially if located outside the US, it is far from obvious that a given movie is available through another service.

Refunding the sale price is still theft.

What did you lose in this theft?

Is there really nothing in your home right now you would be sad if someone took and just gave you the money you paid for it?

Even a digital copy of a movie may not be so easy to replace on the services I have access to.

Stores are not allowed to go home to people and take back the stuff they sold, even if they refund the price. Neither should a company that advertise “pay this price and own this movie” rather than “pay this price and rent it for an indeterminate time”.