Just FYI, I am having the same issue with an already existing and fully-functioning Ark Survival Ascended server that won’t allow me to connect to it today. Now, Ark did get an update yesterday so that is making me wonder if the issue is the version in AMP needs to be updated to match. It doesn’t trigger any updates in AMP though, and I don’t know how to force it to check or force a version update of the game.
Okay, so I stopped my Ark server, changed my AMP settings to “update and start” on startup (not sure if this was necessary) then restarted AMP and it detected a new version for Ark and is currently downloading and installing it. I will report back and let you know if this fixes the issue.