The problem is as stated in the title. I choose the modpack, the console fires up, starts the expected process of loading up all the mods. The client eventually opens, and remains on the Mojang logo as the console goes through the motions. After a while of what looks like regular starting behavior, both the client (still stuck on the Mojang logo) and the console spontaneous close.
There's no crash window or anything, just a sudden and complete closing of both windows.
I can never effectively pastebin the contents of the console because it presumably closes as soon as the relevant line appears, making any pastebin useless. That's my understanding of how it works, anyway, if you think it would help I could try anyway.
The following is the most recent file in the crash-reports folder, let me know if you need anything else: