Update: Login should be working, our testing suggests that it works now.
There appears to be an issue with our authentication token on the forum currently for which we’re investigating a solution. The problem seems to be surfacing for all users when trying to log into SketchUp forum. This could manifest if you log out on your favorite browser and then try to sign in again, but will also turn up if you’re trying to log in to a new browser as well. In short, if you aren’t already logged in, you won’t be able to log in until we fix this bug.
I’ll update this post when the problem is resolved. Tell your friends!