"Failed to sync registry data with server" help with troubleshooting

I've recently been assembling my own custom modpack, meant to be lightweight for friends who lack as good of PCs. There's been one primary issue I've had, and am looking for ways to troubleshoot it, and hoping to learn how to troubleshoot it myself so I no longer have to be a bother to Internet strangers. The main problem I have is:

"Failed to sync registry data with server"

I've looked up the issue online, and most sources say to look for a difference between mods or Forge. I've checked Forge and Minecraft versions and confirmed that they are all the same. I've checked every mod individually and cross-referenced it with the mods I have installed, and found they are all the same versions.

Quote from Josh

It could be that you don't have the same mods or version. But it you have them with the same version, deleting the config folder from the Minecraft installation path might work

Should I do it for server end, client end, or both?

Quote from Roman

Try both

Tried both. It worked. Thanks!