If it helps, I worked around the login issue by logging into my account via a different route first. I logged into Reality Scan (https://www.unrealengine.com/en-US/realityscan) with my unreal account, and then clicked the marketplace tab at the top to work my way back to Fab and then logged into Megascans successfully. I can now see my library (although, oddly, it has a couple of items duplicated) and add new downloads.
There is definitely something amiss with the Fab login though, so Login to the Unrealengine domain first and navigate across to fab so your login comes with you…