Hello, and welcome back to the forum.
First the licence issue.
When you bought the MC23/24 licence, JRiver would have sent you an email from the address "buy-button@jriver.com". Attached to that email would have been an mjr file, with a name like "Media Center24 Windows-123456.mjr". That is your licence file, and if you saved that somewhere on your PC, then double clicked on it in Windows Explorer, MC23 would have been licenced and remembered your licencing when you start it in future. I have always found the "Enter Your Registration Code" method of applying the licence a bit of a pain and prone to keying errors. Using the mjr file has always worked reliably for me.
Now for your forum access issue, all you needed to do if you had forgotten your password was click the Login button without entering complete information (
Green arrow), and then click the "Forgot your password?" link (
Red arrow). Then JRiver would have sent you an email with a link to reset the password, or a new temporary password, with instructions to change it. I don't remember which because I remember my passwords. In fact, I have some software to remember all my passwords for me, and fill in the login screens of lots of sites. I use the free version of LastPass, but there are many others. You may wish to try one.
The "Account didn't exist" message is a bit of a concern. Make sure that you enter your email address accurately.
Now when you upgraded to MC23 it sounds like the upgrade process found your old MC20 Library and brought it over to MC23. But you are correct, the upgrade process doesn't bring the Cover Art folder across as well. If you had all your Cover Art embedded in your media files, you wouldn't have "lost" any Album Covers. But as you haven't done that, you will need to do just a little work to get everything back.
I'll be back with another post shortly for what to do.