I just performed a manual migration of one of my instances. This is a bit technical but should work.
Pre-conditions are:
the immich app instance is on the latest package version
1.55.1
local account (most likely the first and thus admin account) has the same email as the one you want to login with OIDC (this is how immich does the user mapping)
have the app instance id on Cloudron available. That can be seen in the update section of the app configure view in your Cloudron dashboard and looks something like 2c016381-840c-4707-945e-189896fbb3f9
Once those are met, proceed with:
Create app backup
SSH into the server
Run mysql -uroot -ppassword box -e "update apps set sso=1 where id='INSERT-APP-ID-HERE'" - root and password are literal like this, only insert the app id and hit enter
Go to the app's configure view in your Cloudron dashboard and in the location tab simply submit the form without changing anything (this is just a way to make the platform rebuild the container)
Once the app is up, you can relogin with OIDC and the user mapping should work.