How to resolve registry or mod conflict on modded Minecraft server?

A missing record or mod channel error on input may indicate that the client and server are not loading the same list of content.

Step by step

Note the mod ID or save name on the error screen. Examine both the client log and the server's connection line at the same minute.
Compare the Minecraft version, installer and corresponding mod version on both sides. A newer profile may have been accidentally selected in the package manager.
Install the correct modpack on a new client profile. Use a consistent version of the package instead of randomly deleting a single file.
After logging in with the new profile, enter the problematic item or area with control. If only one player is affected, back up player data first.

Deleting world saves or player data on the first try. The error text alone does not indicate that data deletion is necessary.