I am using immersive-portals-0.4-mc1.16.3-forge and preview_OptiFine_1.16.3_HD_U_G4_pre1 (the stable build of optifine has the same issue) and the game crashes upon loading a world. World loads just fine with either on their own. It may also be important to note that the same issue occurs when using the fabric version of the mod in conjunction with a second fabric mod that enables optifine.
I am using immersive-portals-0.4-mc1.16.3-forge and preview_OptiFine_1.16.3_HD_U_G4_pre1 (the stable build of optifine has the same issue) and the game crashes upon loading a world. World loads just fine with either on their own. It may also be important to note that the same issue occurs when using the fabric version of the mod in conjunction with a second fabric mod that enables optifine.
Log: https://pastebin.com/JthpK6pD