One mixin of the common networking module doesn't have the same signature between 1.7.10 and 1.8.9.
I'm surprised previous versions of LFAPI didn't crash in the same way because this code exists since API 1.7.0.
Culprit: https://github.com/Legacy-Fabric/fabric/blob/main/legacy-fabric-networking-api-v1/common/src/main/java/net/legacyfabric/fabric/mixin/networking/client/ClientPlayNetworkHandlerMixin.java#L50-L56
Logs: https://mclo.gs/CLrZjaA
MC1.8+ isn't affected.