Same problem here. Cant run both server and client even with a simple mod template - throwing NullPointerExcetion on title screen on client and net.minecraftforge.fml.LoadingFailedException: null on server.
I make sure I updated mods.toml.
IDEA Community 2018.3
build.gradle
mods.toml
Main mod class ForgeGuard.java
Last line from server log: