Just adding my experiences onto this issue. TLDR; can confirm that the proposed method worked for me.
I was seeing not only this issue, but also "Unable to check for updates" in all update-related sections of the Web UI. My update branch was pointing at 2.7.0-RELEASE, but my system was reporting 2.6.0 and the whole system seemed mildly hosed despite still functioning.
After following the recommendations here by pointing the branch to 2.6.0-RELEASE, then connecting via SSH to bootstrap the package system, I had this:
/root: pkg bootstrap -f
The package management tool is not yet installed on your system.
Do you want to fetch and install it now? [y/N]: y
Bootstrapping pkg from pkg+https://pkg.pfsense.org/pfSense_v2_6_0_amd64-pfSense_v2_6_0, please wait...
Verifying signature with trusted certificate pkg.pfsense.org.20160406... done
pkg-static: warning: database version 36 is newer than libpkg(3) version 35, but still compatible
Installing pkg-1.17.5_4...
package pkg is already installed, forced install
Extracting pkg-1.17.5_4: 100%
Following that, I tried to run an update via the Web UI, but it said the system is up to date. The branch was now pointing at 2.6.0-DEPRECATED or something like that. I switched the branch back to 2.7.0-RELEASE, and checked for updates via the Web UI again. This time, it looked correct and listed 2.7.0 as an upgrade path. I then carried out a full update. After a nerve-racking 5-minute reboot, pfSense came back online and appears to be operating correctly.