I am trying to install industrial foregoing which required tesla core lib. No matter what version of tesla core I download. Server starts up fine, but anyone that joins gets the message below. On single player the mods are working fine. It seems to only be on the server side having the issue. I have even uninstalled and wiped the server and only launched it with tesla core lid and forgelin on it... same issue. So I do not believe its any of the other mods conflicting with it.

[22:03:05] [main/ERROR] [FML]: Unidentified mapping from registry minecraft:items
[22:03:05] [main/ERROR] [FML]: teslacorelib:powder_emerald: 7628
[22:03:05] [main/ERROR] [FML]: teslacorelib:battery: 7566
[22:03:05] [Netty Client IO #1/ERROR] [FML]: Network Disconnect: Fatally missing registry entries

Also, this is for 1.12.2 version of mc running on forge 14.23.4.2760

I have tried:
Forgelin 1.74
Tesla 1.12.201.063
Tesla core lib 1.12.2-1.0.15

Tried a older version of
Forglein 1.6.0
Tesla 1.12.201.063
Tesla core lib 1.12.2-1.0.14

I suspect a version and/or config mismatch between the clients and the server. I suspect that one side is using 1.0.15.13 and the other side is using something older, due to this change in particular: b056d95#diff-9e6f26eae9370cf4016296713ff9607a

I had this problem too, it seems to come from the defaults in the teslacorelib config file being improperly generated on occasion. You can fix it by looking at the debug in your log folder to find what things specifically are causing the problem, and edit teslacorelib.cfg (in your config folder) to register the proper things.