@Tize,
I am not sure what happened, but you should be able to change that value.
I assume that you are logged in as admin, is that correct?
If you retry to set the value and it still does not work, you can safely try to
a) run the command: plesk repair installation
OR
b) use the autoinstaller and: uninstall the backup manager, followed by a (re-)install of the backup manager.
I would recommend option b): it is fast and does not (really) interfere with other components.
Hope the above helps.
Regards....