Files won't start (undefined symbol: gof_directory_async_block_monitor)

Had the same issue.
The cause appears to have been a package conflict causing multiple package upgrades, including `libpantheon-files-core0` (I suppose it was this specific package) to be kept back when using `apt-get`.
Upgrading through `sudo aptitude upgrade` did the following:

Resolving dependencies...
The following NEW packages will be installed:
  backintime-qt4{a} libqt4-help{a} libqt4-scripttools{a} libqt4-test{a} libqtassistantclient4{a} python3-keyring{a} python3-pyqt4{a} python3-secretstorage{a} python3-sip{a}
The following packages will be REMOVED:
  backintime-notify{u} python-keyring{u} python-secretstorage{u}
The following packages will be upgraded:
  backintime-common backintime-gnome gir1.2-freedesktop gir1.2-glib-2.0 gir1.2-gtk-3.0 libgail-3-0 libglib2.0-0 libglib2.0-0:i386 libglib2.0-bin libgtk-3-0 libgtk-3-bin libpantheon-files-core0 midori-granite pantheon pantheon-photos
  pantheon-photos-common

After the upgrade, pantheon-files worked again.

jazzpi (jasper-a):

The aptitude command is not working for me, at least not by default (I guess it's not part of the default installation).

"sudo: aptitude: command not found"

But today I ran the Software Updater (from the Slingshot/Applications menu) instead of upgrading via terminal, and now the Pantheon-Files working again. Thx for the tip!

Had the same issue since running apt-get upgrade && apt-get update yesterday. Running Software Updater (from the Slingshot/Applications menu) solved the problem for me as well.

Judged not to be a bug in Files or its packaging.

This is quite definitely a bug in Files or its packaging, as multiple people have experienced it. I have been experiencing the same issue since I last upgraded a couple of days ago; using aptitude upgrade as suggested by jasper fixed the issue, but it is more of a workaround to have to download aptitude and use that to upgrade, when the usual method is to simply use apt-get.

I think `sudo apt-get -f install` might work as well... But I can't test anymore since I already fixed it.

Still have the problem, renders the OS almost useless. I've tried all recommended fixes.