It happens when the map is being updated. You can unsubscribe and then susbscribe again to redownload the right version if that happens.

thank you

引用自 Docteur Whoa：

It happens when the map is being updated. You can unsubscribe and then susbscribe again to redownload the right version if that happens.

I tried that, but it still had the same issue.
I've also tried :

Uninstalling the map manually by going into my game files and deleting the map (causes a Map Missing error instead)

Entering the map for the very first time (gave a Client Map Differs from Server error, even though it seems it would normally give the Map Missing error. )

Installing the version listed on the website (was older version, most likely wasn't considered to be even remotely the same map as the server's map. gave a Map Missing error if it was the only version installed, gave a Map Differs from Server error if it wasn't. )

Many verification's of the Game Cache (did nothing. )

Any chance that the issue isn't just me being stupid? :(

No you're not stupid don't worry. That error means either you or the server have a different map. If you have the new map and it is still not working, the server you're trying to join did not update, then you won't be able to join it until it also runs the same map.

As mentioned above, this error happens when the map is updated on the workshop. It will solve itself once both the clients and the servers catch up with the update, usually in a few hours.

引用自 Lizard Of Oz：

As mentioned above, this error happens when the map is updated on the workshop. It will solve itself once both the clients and the servers catch up with the update, usually in a few hours.

It seemed to be working late last night, so that sounds about right.
Sorry to bother you, Saxton Heggman.