I tried removing MrTJPCore and its dependent mods (all 6 ProjectRed mods, including Base, Fabrication, Integration, Lighting, Mechanical, and World) from both the client and server, and the same error comes up when trying to connect to the server.
EDIT: I also checked the mods list and none of them appear to be disabled.
latest.log:
https://gist.github.com/lumpman2/7835a098a47aea8f00b32cb36c8f3707