Riding ostrich causes DC:
[INFO] SchindlerShadow lost connection: Invalid move vehicle packet received

avatarmod-1.5.9.2
FML: MCP 9.42 Powered by Forge 14.23.5.2784 Optifine OptiFine_1.12.2_HD_U_C7 110 mods loaded, 110 mods active

Well, that's not good.
-You should have a server log. Can you provide that?
-Have you tried downgrading to 2751?
Other servers don't have this issue. Do you have any mods that change mobs?

Issue was caused by the mod Thaumic Arcana.

Ok cool
Can you report this on their github?

Reported Frontrider/thaumic_arcana#64

Thanks!