@Amanuel_Elhanan in fact your drawing is includes the information Collabora and Reverse Proxy are running on the same host but I’ve missed it…
Take a look at this drawing - it’s for both systems running on the same host but general picture remain unchanged - systems must be able to talk with each other (double check DNS/TLS from each point of view). Depending on you systems are connected there might be unexpected interaction with reverse proxy/firewalls etc as well.