I reset my printer and reloaded software (from HP?) - everything looked successful and the Scanner panel (on iMac screen) changed format from before.
However, I still get the message - The overview scan failed to complete.
If there are extra steps (such as turning printer off and on, or restart the computer), these should be stated.
iMac is 10.11.3 El Capitan (this post panel says the iMac is running Maverics 10.9.1 ?? )
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Mac mini (Late 2012), OS X Mavericks (10.9.1), eMail server on LAN is Dovecot IMAP