We are getting a message that there is a mod mismatch when attempting to connect to the server. We restarted the server twice and had no success. The server doesn't show it's outdated and it's happening to everyone trying to connect.

Any suggestions?

depends on what mods i know few mods i use lately don't update as they should via steam . steam been having issues with ark mods lately few mods that i have necounter this with and had to unsub from mod and resub just to get the updated mod version go into sub items and unsub from any mods that have been updated recently and resub see if that helps

client not server

Sounds like a mod author goofed when they cooked the mod, updating the version number poorly, or regerssing the version number.

Your best bet is to verify your local files and then do a server rebuild. Be patient on both actions as interrupting either of them can cause some unintended side effects. (A rebuild is basically a server side file verify.)

I was getting this error when we had added a new mod to the server but I had not yet personally subscribed to the mod (a poorly worded error message for one that does double-duty like that, it seems).

Oh, it does more than just double duty. That error indicates that something doesn't match between your PC and Server files. Which means of the 30+ use cases that can cause such an error, you get Mod Mismatch.