What version of Premiere Elements are you using and on what computer operating system is it running?
I suspect that you are using a Premiere Elements version earlier than 11.
Those versions earlier than 11 are no longer associated with a Sign In. Ignore or cancel out of any messages that ask
you to Sign In. You will be able to import, edit, and export using the program.
The Sign In was associated with "photoshop.com" which has been discontinued within Premiere Elements 7, 8, 9, and 10.
Also note, that the photoshop.com plus members only content is no longer available.
The Error 400 matter is not a new one. How long have you been working with the program that is giving you this Error 400 message
Please supply more details to help us help you.
Add On...More details on this later if necessary, do not misunderstand any program messages that you may encounter; you do
not have to join Adobe Revel in order to use your Premiere Elements 7, 8, 9, or 10 program.