Hey @martin49, welcome aboard the Community!
From what you mention, there’s a high chance you’ve set up a brand new Dropbox account, which is why you don’t see your files when logged in to it.
The computer that still shows your old Dropbox files, does it also show your paid account and previous email address? You can check this this way and then compare that email address to the one you see here. Are they the exact same?
Let me know what you find.