Thank you! I indeed tried installing Optifine a while ago before I had Forge, but I thought I deleted it. And now it appears there where indeed some files left. I deleted them and now it's starting up again!
I got Forge today, and I ran into this error code when trying to boot up Forge and now I cannot start it up. Link to the Bug report and Debug log is below.
https://pastebin.com/0L3HHXrk