I keep getting this problem too, actually. I'd love to get a fix, want to use mods. :)

Shut server down, Delete all mod files from the mod folder on the server, Then restart using update & validate......

Client, unsubscribe to all, goto mods folder delete everything in there, rejoin server so its the server that registers your download.....

try that

Well, tried that, got passed the mismatch message and downloaded the mods, but it got stuck at loading primaldata, booted me, and now is giving me the error again. =/

Thats a BAD file installation....PrimalData is the bit it loads when your say stuck were you died, or if your somewhere you shouldnt be or the server still thinks your body is somewhere it isnt.... amoung other things....

Okay, got one mod loaded and was able to enter the game. I restarted the my PC and revalidated. It seems that the program has a problem with letting files go after closing so I'm not surprised something got corrupted. Thank you for the assist, hopefully this will help the OP.

(Also, you have to be careful about load order, I just moved one mod to the front and was able to load it without problem when I was struggling before.)

Yeah Load Order is probably thee most important thing lol, Always has been