PaperMC/Waterfall#366
shedaniel/RoughlyEnoughItems#74

When mod with packed Fabric API is installed, i unable to connect "custom-core" servers (BungeeCord, FML, "BeeCord" (hivemc custom core)), vanilla, or just plain Fabric Loader is fine, however.

After "Encrypting", on "Joining World" connection interrupts with reason
Internal Exception: net.minecraft.net_class151: Non [a-z0-9/._-] character in path of location: minecraft:BungeeCord
Internal Exception: net.minecraft.net_class151: Non [a-z0-9/._-] character in path of location: minecraft:FML

Happens on 1.14 and 1.14.1

Technical Info:

Launcher: MultiMC5 0.6.5-develop-1308
Fabric API version: v0.2.7+build.127 (in "modpack" included with Autofish, i-am-very-smart, REI)
Mods:

  auto-config-1.0.1+mc1.14
  Autofish-0.8.2-fabric-mc1.14
  BBOutlineReloaded-1.0.11-1.14-fabric
  blockmeter-1.0.1
  ClothEvents-0.3.1.23
  CustomSelectionBox-3.2.0
  durabilityviewer-1.14-fabric0.2.7-1.5
  easierchests-1.14-fabric0.2.7-1.3.1
  FabricProxy-1.0.0
  Hwyla-fabric-1.14-1.9.16-60
  i-am-very-smart-1.0.0+mc1.14
  malilib-fabric-1.14.0-0.9.5
  metatips-1.1.4+1.14
  ModMenu-1.5.3-84
  mousewheelie-1.1.5+1.14
  REIAddons-1.1.0
  RoughlyEnoughItems-2.9.110
  tweakeroo-fabric-1.14.0-0.9.3

Logs

Fabric api 0.3 should fix this, can you test?

fabric-api-0.3.0-pre+build.156.jar (latest version from curseforge)

Same excepetion:

Logs
MultiMC Console Log

Mods:

  auto-config-1.0.1+mc1.14
  Autofish-0.8.2-fabric-mc1.14
  BBOutlineReloaded-1.0.11-1.14-fabric
  blockmeter-1.0.1
  ClothEvents-0.3.1.23
  CustomSelectionBox-3.2.0
  durabilityviewer-1.14-fabric0.2.7-1.5
  easierchests-1.14-fabric0.2.7-1.3.1
  fabric-api-0.3.0-pre+build.156
  FabricProxy-1.0.0
  Hwyla-fabric-1.14-1.9.16-60
  i-am-very-smart-1.0.0+mc1.14
  malilib-fabric-1.14.0-0.9.5
  metatips-1.1.4+1.14
  ModMenu-1.5.3-84
  mousewheelie-1.1.5+1.14
  REIAddons-1.1.0
  RoughlyEnoughItems-2.9.110
  tweakeroo-fabric-1.14.0-0.9.3

Attempt №2: Disabled mods that include old version of Fabric API (Autofish, i-am-very-smart, REI)

Still the same
Logs
MultiMC Console Log

fabric-api-0.3.0-pre+build.156 and fabric-resource-loader-v0-0.1.0+f1618918 in gradle: gradlew runClient works fine and I can connect to hypixel. gradlew build, mod and fabric-api-0.3.0-pre+build.156.jar to .minecraft/mods I get the same error as in the screenshot above when trying to connect.

Removing fabric-networking .jar from fabric-api-0.3.0-pre+build.156.jar/META-INF/jars (and others that use it, like fabric-registry-sync etc) again allows me to join hypixel. Can't say for sure which of them fixes it, as a bunch have to be removed at once to even be able to launch

So fabric networking should use some sort of fake identifier for packet id instead of the strict java edition identifier.