It happened again with 60 updates that showed up in the update notifier.
I selected show updates, then clicked on install updates, I was prompted for my password, the update-manager went gray, after a minute everything became normal again but the 60 updates weren't installed, so I clicked on install updates again, but after 5-6 minutes got the error in the attached image.
After this I ran as root:
apt-get update
apt-get upgrade
apt-get dist-upgrade
and all the updates got installed.
So concluding I can only install updates with apt-get and not with update-manager.
It happened again with 60 updates that showed up in the update notifier.
I selected show updates, then clicked on install updates, I was prompted for my password, the update-manager went gray, after a minute everything became normal again but the 60 updates weren't installed, so I clicked on install updates again, but after 5-6 minutes got the error in the attached image.
After this I ran as root:
apt-get update
apt-get upgrade
apt-get dist-upgrade
and all the updates got installed.
So concluding I can only install updates with apt-get and not with update-manager.