Server Authenticaton Error

Servers now require a login via SLG account. Your game server provider needs to update their systems to allow you to enter these credentials.

The reason is the same as for why Valve requires you to get a token from a Steam account with verified mobile phone number to host a Counter-Strike or similar server - enforcing Terms of Service. Hence an opt-out is not possible.

Yes, it works without account in offline mode.

Of course both the server and the client need to be in offline mode, the mode is intended for when you are actually offline.

You need to either use -username= and -password= OR -userToken= with a token from play.eco in the command line parameters to start a server. If your host hasn't provided means to add those yet, you should reach out to their support to make that available. Configuring the login via config files is not possible.