One of your options is to stop using one of the two mods. Once it crashes, you'll be able to identify which mods is in conflict with another one. Uninstall all of your mods (forcing an update is a good way, but you'll need to re-install Forge) and re-install them all one by one. Most of the time, conflicting mods does not work because they edit the same bit of data in the game files and the mods that is last loaded overwrite the data of the first mod which cause the first mod to crash since what it need is not there or not correctly programmed. If you have mods installed, the error could be caused by conflicting mods, which are two mods that are not able to work if another one is installed. Force an update on Minecraft and re-install Forge by following the exact steps. If you have no mods installed and only Minecraft Forge, it could mean that you did not install it correctly.

There could be a lot of cause for this error. This means there is an error somewhere in the programming where a variable is not found or something similar. What is happening is that you have a Null Pointer Exception. I do not know if you have mods installed or not, so I am going to give you a solution for both situations.

Minecraft forge launcher frozen