There seems to be an error when a player attempts to join a server running S+ for the first time. For some reason, S+ is not auto-downloading for some people, and instead of just getting a timeout error, they are getting this:
What you need to do is exit Ark, go to the Ark Survival Evolved workshop in Steam, and subscribe to S+ Structures Plus manually. After you do so, exit Steam and restart it. Once in, load Ark, but DO NOT JOIN A SERVER. Once at the title screen, you should notice S+ downloading the the bottom-right of your screen - when complete, enter the server.
(If you do not see it, give it a few minutes, and then try to enter the server again. If it fails, exit Ark, restart Steam and go back to the Workshop and confirm you have the mod subscribed - the only time this failed to work for us so far is when a player made an error and didn't subscribe to the proper mod).
If you have any further issues, please do let me know!