I try to restore a backup from ZIP.
I choose to restore plugins.
After click on "Restore" button Jenkins enters quiet state and hangs.
The console shows "Cannot find plugin 'Hudson core' with the version '1.651.3'." Please note that this is the version of running Jenkins.
Jenkins remains in quiet state, ie. shows "Jenkins is shutting down" on all pages.
A restart of Jenkins is required, ie. close console and start Jenkins again.

Dont know if anything would be lost, as the saved configuration remained unchanged during tests.