it keeps crashing whenever I join my multiplayer world. Optifine is only installed client sided! And I did not pick any shader yet, its just basic Optifine 1.16.4_HD_U_G6 (which supports Forge .35 and up I think so there should not be any problem there). The Crash report says something about the Flamethrower state.
Crash report can be found here: https://pastebin.com/R71QUPFX

I also met with this problem -- it seems to happed occasionally.