Hey there I went to add this mod to ATM7 and single player worked great and the server loaded just fine but getting this issue on joining.
https://paste.in/Jht9XN

Which version of Forge is the pack using and which mod adds constantan?

37.0.44 and we added the thermal series ingots and blocks to All The Ores https://www.curseforge.com/minecraft/mc-mods/ato/files/3439307

I pushed a new version fixing tags related to copper. Realized I can just remove them since it's not a vanilla resource :S