It could be something else
The Graphical PCBSD has a pretty generic message. I also had the same problem, but discovered the true issue when I ran the update from command line:
start here (likely requires root or sudo):
I believe that you need to run check (pc-updatemanager --check) which will tell you what upgrade command to run with an absolute path.)
When I ran the actual update via command line I found out that I had insufficient disk space, and had to delete a few superfluous jails.