Sorry that you've had such a frustrating experience. This isn't Cyberlink tech support or customer service, it's a forum of fellow users of Cyberlink products like you.
From your description, it seems like you might want to try clearing your browser's cache and deleting any and all saved passwords for the DZ website. Search online for the specifics for how to do that for your particular browser.
You may also want to try a completely different browser. Download one, or use Internet Explorer that's still built in to Windows and see if you have a more reliable experience. Personally, I have not had any problems with DZ or any Cyberlink logins, and I typically use Edge.