Thomas Trout Young

I have the same issue

Here's a work around. I found out.

Check the version of that specific modpack, then select and play the vanilla version ON the same launcher as the modpack. As soon as you get a successful run with the vanilla version. Close it, then open up the modpack and play it. If all goes well, it should work as normal.

My best guess as to why the modpacks wont work upon first play is because the new launcher refreshes version files, and deletes old launcher configs, therefore the version files for the game don't load, causing a crash.