It's an issue with Mods portal-Bethesda is working on it. I've had the same issue on ps4, but was eventually able to log in.
Which account are you using?
You need to make an account for the new forum: https://beta-community.bethesda.net/
it's been doing that to me as well won't let me in now for a week is there a way to fix this I had unstalled it then reinstalled it reset the Xbox one and then moved the Skyrim onto my external drive then I deleted it to move to the internal one and it still won't let me log in