So far, every time I've seen this error with a customer it was because of an expired token, and/or the new token not being properly configured (wrong Composer config file, basic instead of bearer, etc.).
But we can't exclude infrastructure issues with the marketplace.
Unfortunately I don't have a lot of hands-on experience, since I don't actually use any premium extension from the marketplace myself 😅 My own forums use the development version of my extensions straight from GitHub.