I have vaultwarden installed on my home assistant, befor on a docker. All run without problems only with windows software it doesen’t show any passwords.
I use the nginx proxy manager also on home assistant.
When I try to go on xxx.mydomain.xx the side load and nothing happen. Also the program shows no passwords.
On my android and outside from the local network all work like a charm with the same domain.
If you are using the same domain internally and externally you may want to look into split brain DNS also known as Split horizon DNS
Presuming as you said it works externality but not when connected internally, may also be how your NGINX proxy is handling the data from your internal LAN vs external connections?
If you aren’t seeing anything load inside your network I’m guessing you need to setup hairpin nat on your router. If you get a login page on both but don’t see passwords on one you may be running two instances of Vaultwarden behind your proxy and getting different upstreams depending on the domain name you use.