I've been getting this error when trying to join my friend's modded server. I understand why I should be getting this error; The client and the server do not have the same mods or version of the same mod.
However I should not be getting this error considering me and my friend are using the exact same copy of the mod (Recurrent Complex). I sent him the .jar file of the copy I was using. Furthermore, when he deleted the mod from the mods folder and restarted the server, I still got the same error, even when creating a new world without the mod installed.
On top of that, when looking at the server in the server list, it says that there are 4 mods that are incompatible, but the error only lists one.
Also here is the server log https://pastebin.com/UMm1LVyh
Client log too https://pastebin.com/Zr8S7SB6