I was using an iron microwave in the game and it caused the server to crash and shut down.
After connecting to the server, the server will crash and shut down.
game ver. 1.20.2 ,mod:fabric ,end code:-1073740791

there's not enough info here, I need a crash report, exit codes are almost always meaningless

I also ran into this same crash that caused my server to go down. It happens when a non-smeltable item, including nothing being in the slot, is put into the iron microwave and the start button is pressed. This also happened when I went into a world to test it. I can provide a full crash report if necessary! I would also like to add that it will not allow anyone to go near the iron microwave, and if anyone who was in the chunks near it logs back in, it proceeds to crash.

The game crashed whilst rendering block entity
Error: java.lang.NullPointerException: Cannot invoke "net.minecraft.class_8786.comp_1933()" because the return value of "com.unlikepaladin.pfm.blocks.blockentities.MicrowaveBlockEntity.getRecipe()" is null