Howdy! we recently had a small lan for our clan, only to discover we could not play ark.
our ark server only allowed for ONE person to log in with the IP (Note: They are all on their own steam accounts, all with valid purchases of ark).
When somebody would connect, it kick the previous person off the server, and allow the new person on; This repeats when the next person logs on.
Edit: never-mind, turns out it was the router.