Go to System -> Updates Sites and click the Rebuild button.

Try and enter the key in again.

Ok, I tried that. It did not help, still get 500 error when trying to save.

Please send me a login - https://www.joomlacontenteditor.net/contact/site-login

I was able to update the key using the Update Sites method in Joomla 4 - https://www.joomlacontenteditor.net/support/faq/subscription/using-the-subscription-key

I am not able to determine the cause of the 500 error. Do you have access to the site error log?

I am sorry I did not research... I likely could have found that myself. I'm sure I do have access to the logs, or I can get the vendor to provide them. Would you like to see them?

Since we're here, though.... the pre-update check is complaining about "System - JCE JCE Pro Extension Package". Can I safely ignore that, or should I disable something before I proceed with the upgrade? I have much more to do, so I am not in a rush for an answer. I should mention, in case its not obvious, this upgrade issue is on a different site!

the pre-update check is complaining about "System - JCE JCE Pro Extension Package". Can I safely ignore that, or should I disable something before I proceed with the upgrade? I have much more to do, so I am not in a rush for an answer. I should mention, in case its not obvious, this upgrade issue is on a different site!

I'm not seeing this in a pre-update check on a test site. This is most likely due to an older cached version of the manifest file being used for the check, and it is nothing to worry about. You can safely proceed with the Joomla update with the latest version of JCE Pro installed.