Hi @focus!
We’ve not seen that issue on Tutor versions 12.0.2 or 13.1.4 when logging in with a username and password when OAuth2 is enabled. I publish our OAuth2 setting in post Social login configuration for edx as plugin in tutor a while back. Perhaps you can view our config settings to see if anything is different that might trigger the login-service-client-id error that you are seeing.