Problem

Wenn a customer-specific value is it can not be reset any more. So if the default set is empty (e.g. runtimeOptions.lengthRestriction.maxNumberOfLines) and this value is set for a certain customer, this setting can not be removed.
The following error is thrown

Your data could not be saved, on saving there was a conflict on server side!
Server Message: Configuration value invalid: not a valid integer ''

addition to that

the above value can be set to a negativ integer like -3 which makes absolutely no sens here.

Solution

Add a button "EN: Remove client specific config - system default will be used again" / DE: Kundenspezifische Konfiguration löschen – Systemstandard wird wieder verwendet".

Show this button only for configs, that are overwritten on client-level.