Game will not launch if both multiconnect & optifine/optifabric are installed alongside immersive portals. It works with just optifine and immersive portals, or just multiconnect & immersive portals, but not all three.

Minecraft 1.17.1
Java 16.0.1 (AdoptOpenJDK 64 bit)
Fabric Loader 0.11.6
Fabric API 0.39.2
Immersive portals 0.20
Multiconnect 1.4.12
Optifabric 1.11.20
OptiFine G9 pre35

Immersive portals on curseforge: https://www.curseforge.com/minecraft/mc-mods/immersive-portals-mod
Latest.log: latest.log

multiconnect is not compatible with immersive portals because it changes the protocol.