Comment 13 for bug 1576800

Foreword:
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I cannot tell whether printing and scanning on the network attached MFP device would have worked out of the box using the repackaged HPLIP software 3.16.3 which comes pre-installed on Ubuntu 16.04.
I have not tried, because the HP website had told me that for full support of HP Color LaserJet MFP M277dw I needed HPLIP 3.16.5 or above.

Root cause:
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The hplip-installer which comes with the genuine HPLIP software tries to uninstall any pre-installed HPLIP software on the system, which it finds, before starting to install iteself, ... and fails to do so!
In my case this was the root cause why after installing HPLIP 3.16.11 printing worked, but scanning failed.
HPLIP 3.16.3 had not been uninstalled completely.
The reason for this failure seem to be 2 things:
+ The HLPIP 3.16.11 installer had not identified the existing HPLIP software packages
  properly and therefore not uninstalled all hplip packages which Ubuntu pre-installs.
+ The HPLIP 3.16.11 installer had completely ignored the separate package named
  libsane-hpaio, which enables access to the scanner.

Mixed HPLIP software packages:
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As a consequence after installing HPLIP 3.16.11, there existed a mixture of HPLIP 3.16.3 and 3.16.11 software on the system.
And most important, the libsane-hpaio library was present in 2 different places.
And guess what: the outdated libsane-hpaio was found instead of the recent libsane-hpaio.
This was what prevented the system from accessing and using the networked scanner.

Details:
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The Ubuntu provided libsane-hpaio 3.16.3 had been installed in the directory
+ /usr/lib/i386-linux-gnu/sane (32-bit system)
The genuine HP libsane-hpaio 3.16.11 had been installed in the directory
+ /usr/lib/sane
and was ignored.

Solution:
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Before re-installing HPLIP 3.16.11 from scratch, used Synaptic to make sure the following packages had been uninstalled:
+ libsane-hpaio
+ hplip-data, hplip-doc, hplip-gui, hplip
(all version 3.16.3+repack0-1)

Next uninstalled HPLIP 3.16.11 completely by executing "sudo /usr/bin/hp-uninstall"

Next re-installed HPLIP 3.16.11 by executing "hplip-install" in directory ~/Downloads/HPLIP/hplip-3.16.11 one more time.
hplip-install will later on offer to execute hp-setup.
hp-setup will permit to download and install the needed HP plugin, which in turn also reinstalls libsane-hpaio.

Finally tested access to the scanner from inside the "HP Device Manager" GUI and from simple-scan.

Result:
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Printing and scanning finally work as expected.