So, all our mautic instances updated fine. Some of the themes might have to be removed manually since it seems the config.json changed from mautic 3 to mautic 4.
If some pages error, check the logs in System info i.e the Logs view in https://mautic.domain.com/s/sysinfo. You might see an error in the logs like so:
[2022-01-25 15:07:18] mautic.CRITICAL: Uncaught PHP Exception Mautic\CoreBundle\Exception\BadConfigurationException: "Theme Blank not configured properly: builder property in the config.json" at /app/code/app/bundles/CoreBundle/Helper/ThemeHelper.php line 673 {"exception":"[object] (Mautic\\CoreBundle\\Exception\\BadConfigurationException(code: 0): Theme Blank not configured properly: builder property in the config.json at /app/code/app/bundles/CoreBundle/Helper/ThemeHelper.php:673)"} []
To fix the above, open the file manager:
Go to themes directory
Delete the blank-grapejs and confimme (sic) theme directories. You might have to delete more directories depending on the error messages you see. The above error gives a hint as to what theme is causing a problem ("Theme Blank not configured properly")
Thanks @luckow for helping us debug.