My server just crashed after receiving the same error repeatedly. Not sure what could be causing that or how to fix it. Please let me know if you have any ideas on how to fix it as my players are getting annoyed.

https://gist.github.com/Rithenar/0b7f873e652b9eb1851bec4511cbc19f

You properly missing the Mod.

Sorry, what does that even mean?

had the same issue, after updating ATM to 1.3.8.
The exception means that the class could not found, thinking he isnt loading the mod.

I'm getting this same crash playing the Direwolf20 1.16.4 pack (pack version 1.3.0)

https://pastebin.com/8S0MqJTA