ALL gadgets went missing after a Restore System activity.
Error message as follows:

Tried to add gadget but shows only 1 entry:

I checked Add-on Manager and still see gadget plugin still there with most of the modules enabled:

Checked the logs and see error messages such as follows:
2017-05-21 03:59:38,040 http-bio-8080-exec-25 WARN anonymous 239x2403x1 - 192.168.48.231,192.168.85.122 /secure/Dashboard.jspa [gadgets.renderer.internal.GadgetSpecFactoryImpl] Error occurred while retrieving gadget spec for rest/gadgets/1.0/g/com.atlassian.jira.gadgets:introduction-gadget/gadgets/introduction-gadget.xml
It shows that for all the missing gadgets.
Our JIRA version is 6.3.1
Can anyone tell what seems to be the issue here?
Any help is much appreciated.
Gideon