Strange! This is happening because the game is finding a discrepancy between the version the players have installed and the one on the server. So the problem is somewhere in there. (I have had no issues playing my campaign on a dedicated server recently, but I have had the issue you are describing with other workshop campaigns after the Last Stand update. This is in part why I have been working on a new version so that any inconsistencies are solved.)

It sounds like you've already worked the issue a bit to double check that the versions are the same, but there are a couple things you could try. There are two ways to get the campaign currently and that is from the workshop and from Gamemaps. These VPKs should be identical (keyword: should, they are literally the same file). However, the way the workshop loads, installs, and registers the campaign in-game, rather than just being automatically read from the addons folder like manually downloaded installed campaigns may be introducing the inconsistency you are seeing.

You could try different combinations of having the workshop or gamemaps VPK on the server, or being used by the players. (ex: server & players = workshop version, server & players = gamemaps version, server = gamemaps, players = workshop, etc). Last thing I would double check is to make sure there are no conflicts with other addons (which it sounds like you have already done) not just on the player side but also server side, including SourceMod stuff.

Alternatively, if all else fails, and all the players have exactly the same version, installed from the same location, one player could try hosting the campaign lobby locally, rather than using a dedicated server.

Thanks for the help, I think we may have solved the issue but won't know until tomorrow night when we get to test it properly, if not I will run through these tests you suggest.

Appreciate the reply :)