SoF and Haema both use hungermanager. Becoming a vampire normally isn't the issue on itself if not choosing it as origin as this would lead to (integrated)server crash on world join.
Maybe adding compat/config option to disable it on per-player basis or some other solution to fix?

java.lang.NullPointerException: Ticking player
at Not Enough Crashes deobfuscated stack trace.(1.16.5+build.9)
at net.minecraft.entity.player.HungerManager.handler$che000$onDeserialize(HungerManager:574)
at net.minecraft.entity.player.HungerManager.fromTag(HungerManager:89)
at com.williambl.haema.VampireBloodManager.fromTag(VampireBloodManager.java:158)
at net.minecraft.entity.player.PlayerEntity.checkBloodManager(PlayerEntity:33906)
at net.minecraft.entity.player.PlayerEntity.handler$fji001$vampireTick(PlayerEntity:33853)
at net.minecraft.entity.player.PlayerEntity.tick(PlayerEntity:274)
at net.minecraft.server.network.ServerPlayerEntity.playerTick(ServerPlayerEntity:434)
at net.minecraft.server.network.ServerPlayNetworkHandler.tick(ServerPlayNetworkHandler:219)
at net.minecraft.network.ClientConnection.tick(ClientConnection:232)
at net.minecraft.server.ServerNetworkIo.tick(ServerNetworkIo:173)
at net.minecraft.server.MinecraftServer.tickWorlds(MinecraftServer:882)
at net.minecraft.server.MinecraftServer.tick(MinecraftServer:811)
at net.minecraft.server.integrated.IntegratedServer.tick(IntegratedServer:91)
at net.minecraft.server.MinecraftServer.runServer(MinecraftServer:670)
at net.minecraft.server.MinecraftServer.method_29739(MinecraftServer:257)
at java.lang.Thread.run(Thread.java:748)