BungeeCord and Waterfall seem to send invalid plugin channels for 1.13+ clients when 'forge-support' is enabled, potentially causing issues with Fabric API
Reproduction:
Install https://github.com/ViaVersion/ViaFabric, enable client-side mode and try to enter a 1.13 server with BungeeCord with 'forge-support' and get kicked

That is not a Fabric issue, though. The crash, for all I can see, is because legacy packet channel names are not translated to Mojang's spec. Report to them, probably trying with Paper's fork first as they might be able to guide you better.