Play on local hosted World (Offline Mode) While Steam is Online

That is not possible. (Games started through the client don't need a registration, though, so you should be able to play just fine by starting the world locally through the client.)

Yes, that is intended behaviour - online servers need to be registered. Worlds started through the client will still shutdown when the client is closed.

I'm sorry you are unhappy about that. To allow us effective enforcement of our Terms of Service it is now mandatory to link servers with a login - similar to other games, for example Valve games (Counter Strike, etc.) requiring the creation of a unique token linked to a mobile-phone verified Steam account. Eco will also support the creation of tokens as an alternative to directly logging into an account on the server as well.

The same behaviour is reflected in Eco, if you host a local game through the client or put the server in offline mode (-offline) you do not need a registration. Dedicated servers that are online are connected to the internet and as such can be joined by anyone the administrator desires, we have no way to prevent them changing who can join once they are started, as such they need to be registered.

As for the Steam linking issue, we are working on that, please refer to the following Discord announcement:

PSA for those that get error 500 when trying to link with Steam:

We found the issue, as a workaround instead of logging into your created account just click on "Log in with Steam" when opening play.eco. It will then ask you to create an account that should work.

If you don't want to do that we'll have a solution for the existing accounts on monday.

I'll bring that up to the team :)

There is no progress saved on our servers and the marketplace items should be cached.