How do I shutdown Teamcity?

I can't reply to suamere's comment but just like the shutdown script there is a startup script in the bin folder, e.g.

D:\TeamCity\bin\startup.bat

I don't think this is an issue on the TeamCity side, it's more likely that the error message is correct - the username doesn't exist. The steps you're following look fine, have you double / triple / quadruple checked the username is spot on? Maybe also check whether you can change the password for other user names and see if it's an isolated problem or not.

Shut down TeamCity via the command in its bin directory:

D:\TeamCity\bin\shutdown.bat

in the Teamcity/bin directory run the following

./teamcity-server.sh stop