Since you are running on premise, I am guessing you can access and change the database.
- Stop application
- Query database for User records that are named 'mxadmin' with any casing (!!!)
- Delete these records
- Start application
Log in, using correct casing for MxAdmin.
Mendix is able to get in a state where it screws up the casing for the MxAdmin account in the database. Might be the case here, might not be the case here, but better to be safe.
I finally figured it out. The code was all good but the /etc/nginx/conf.d/[appname].conf nginx config file had a bad IP address. Once the file was updated and nginx was restarted I was good to go.