Mac OS X El Capitan
This ordeal began when I was unable to connect Time Machine to a newly purchased Time Capsule; it still wanted to hang on to an older external drive which had died. I called Apple and they instructed me to create a new user account and try connecting the Time Capsule. This worked and I was able to back up to Time Capsule.
But. Reinstallation of Google Chrome extensions fails. First I see a dialog box I didn't see before:
Then when I click 'Save', it displays this:
And in the download bar, the item is tagged "Failed -- insufficient permissions".
Extension installation fails for Safari also.
I tried booting into the recovery partition and repairing the user permissions, but that did not solve the problem.