For what its worth, I haven’t used it in a while and tried to re-set it up after seeing these notes - and it is working.
There have been many changes in my system since my posts above, but most notably I’ve set up a Caddy plugin on OPNSense to provided for secured access to bitwarden. Caddy on OPNsense also manages the letsencrypt certificate renewals - so that is good too.
Hi, yes I am using nginx the phone is on the same network not on a VLAN and the self hosted server has WAN access and I connect with https://mydomain.com
Since it’s accessible on the WAN side via your domain I’d be interested to know if it works if you turn WiFi off on your phone then try via the phone app so it’s not trying to route internally…
Hi Alphanaut good thinking 
, I turned the WIFI off on my phone and tried to connect via GPRS but still the same “An error has occured”
@ringo1 What is you exact server version now?
And did you try to to delete the BW app data on your Android phone (via the Settings → Apps in Android)?
Hi Nail, sorry for the late reply the site would not let me reply as it said i had reached the maximum number of replies a new user can create on their first day
In answer to your questions:
Server version: Vaultwarden 1.35.3
App deletion: I deleted the app via app management in play store
However good news after checking the vaultwarden hub page there is an update to 1.35.4 been up for 2hrs. Using the new image I can connect to the server with the Bitwarden app.
Thank you for all who tried to help me.