Minecraft v1.21.1
NeoForge v21.1.172
Horseman v1.4.2

There appears to be a compatibility issue between this mod and Two Players One Horse due to an entity mixin class being set twice for a single horse. This was reported to the dev for TPOH, who suggested that safeguards on HM should be applied to prevent this error from occurring.

Here is a link to the issue for reference: Beethoven92/TwoPlayersOneHorse#1

Betthoven92 was able to address error from there end to prevent the desync that was occurring, but the server is still throwing errors when interacting with horses. Here is the relevant log for review.

Server Error.txt

I've added some checks to not set the attribute twice to the new version. But I cannot properly check it right now as curseforge doesn't let me download the TPOH mod into my dev env. So please let me know if it's fixed.

It would appear that the errors have been fixed! Thank you for the assistance and quick response.