I was having the same problem. I couldn't edit the backup job.
I noticed there were some old backup tasks running stuck at different percentages so i shutdown the appliance, unmount the disks of the machines that were being backed up by the appliance, and restarted the vCenter services to get rid of all the stuck tasks.
Once i did all that, i started up the appliance and i could edit the job. I still can, but i'm under the impression that eventually the problem will come up again...probably related to unfinished backup tasks being stuck...
I'll keep on testing...