There are a few potential fixes if you are unable to use a specific email account's Roundcube webmail because it is stuck at the "Logging you in" screen:
1. Delete the cookies and cache in your browser: Frequently, problems with websites that are not loading correctly can be resolved by clearing the cache and cookies in your browser.
2. Browser extensions should be disabled because some can affect websites' performance. Try turning off all extensions before attempting to use Roundcube webmail once more.
3. Check the email account's storage limit because it could be challenging to access Roundcube webmail if it has reached that limit. You may determine the email account's quota by connecting to your cPanel account and viewing the email account's usage data.
4. Verify the settings for the email account: Verify that the cPanel account's email account settings are set up correctly.
5. Look through the server logs: If the problem continues, you can look through the logs to see if any error messages about the email account or the Roundcube webmail are present. The "/usr/local/cpanel/logs" directory is typically where the logs can be found.
6. Contact your web host: For more help, you might need to contact your hosting company if you're still having trouble accessing Roundcube webmail. They might be able to identify and resolve the problem.