<sorry for my english grammatics errors if they exists>
I can't login in my Scratch account. When i'm trying to login, loginbox does nothing and does not show any errors. When i open the Network Inspector,
it shows me error 400 Bad Request:

For some reason I'm authorized at forum.
My browser / operating system: Windows NT 10.0, Firefox 77.0, Flash 32.0 (release 0)

That's strange… I'm afraid I can't do anything to help but sometimes I find this is for some completely silly reason like being in full-screen, so maybe check for any minor changes to what you normally do. Maybe close the window and start again?

The 400 (Bad Request) status code indicates that the server cannot or will not process the request because the received syntax is invalid, nonsensical, or exceeds some limitation on what the server is willing to process. It means that the request itself has somehow incorrect or corrupted and the server couldn't understand it. The server is refusing to service the request because the entity of the request is in a format not supported by the requested resource for the requested method . Therefore, it prevents the website from being properly displayed. The main thing to understand is that the 400 Bad Request error is a client-side error.

The cause of a 400 error can be a wrongly written URL or a URL that contains unrecognizable characters. Another cause of the error might be an invalid or expired cookie. Also, if you try to upload a file that's too large. If the server is programmed with a file size limit, then you might encounter a 400 error.

http://net-informations.com/q/mis/400.html