When it tries to check for mods, it is doubling up a bunch of mods in the cache. it then tries to load all of those doubled mods and refuses to start when the second version of each mod is missing from the actual mods folder.
edit: and going back to the old version, its still complaining about mods being removed but not uninstalled... Oh boy.
edit 2: managed to get it fixed and working on the old verison with only losing a couple mods that I had to re-install.
Would NOT recommend updating.