cubicweb #2793190 upgrading from 3.15.9 to 3.16.1: 'CWUser' object has no attribute 'prefered_language' [rejected]

I am attempting to upgrade a cubicweb-brainomics instance from CubicWeb 3.15.9 to 3.16.1.

On my Ubuntu 12.04 workstation, CubicWeb 3.15.9 had been installed using Ubuntu packages.

CubicWeb 3.16.1 has been installed from the tarball. Note that CubicWeb 3.15.9 has not been uninstalled. I believe this is not be a problem: as far as I can see CubicWeb 3.16.1 files should always be used over CubicWeb 3.15.9 files. Indeed CubicWeb 3.16.1 has been installed under the `/usr/local` root which has precedence over the `/usr` root of CubicWeb 3.15.9, both in terms of shell `PATH` and most importantly Python path.

After upgrading, my first attempt to run my cubicweb-brainomics instance fails as expected:

$ cubicweb-ctl start -D localizer
instance localizer not started: instance has cubicweb version 3.15.9 but 3.16.1 is installed. Run "cubicweb-ctl upgrade".

Now when trying to upgrade I get this error message:

$ cubicweb-ctl upgrade localizer

Upgrading the instance localizer
Traceback (most recent call last):
  File "/usr/local/lib/python2.7/dist-packages/cubicweb-3.16.1-py2.7.egg/cubicweb/server/", line 246, in __init__
AttributeError: 'CWUser' object has no attribute 'prefered_language'
instance localizer not upgraded: 'CWUser' object has no attribute 'prefered_language'

How do I debug this? Is this a CubicWeb issue or is something wrong with my setup?

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