I see a warning saying "You have data stored in an older format and/or unreadable data." with two buttons "Manage" and "Dismiss", and/or "It is acceptable to leave unreadable data in these files, as Jenkins will safely ignore it."
In most cases, these messages can be dismissed. This can occur when you configure a plugin, and later disable it.
There are jobs using the disabled plugin’s functionality, and the data is still in config.xml but has no purpose without the plugin.
Selecting the "Dismiss" button may be appropriate if the plugin was temporarily disabled, or is intended to be used again in the future.
If you click the "Manage" button, you will be able to delete the keys that have no purpose in the current context - in the config.xml for each of the listed jobs.