I recently had a similar problem where I had changed my password using the webadmin tool, but even though I was logging in to the plastic client with the correct credentials, it kept failing and prompting for another login. This persisted even though setting the client name/password using cm configure worked fine.
It turned out the profiles.conf file in the %localappdata%\plastic4\ folder for some reason had cached bad login info. I deleted the entries from the profiles.conf and then I could log in and everything worked.