mismatched

What is the full and exact error message you get when you try to join the server?

The error message "failed to synchronize registry data from server" typically appears when the installed mods on the player's computer do not match the installed mods on the server.

For modded servers, players must have the exact same mod collection installed on their computer as the server.

It is important that the version of the installed mods is identical as well as the version of Forge itself!

If you have so called client-only or server-only mods installed, it may help to temporarily remove them.