Hi,
I ran into this problem and had spent some time trying to figure out what really happened.
I had:
Windows 7 Pro SP1
3DS Max 2014 SP5
a scene with character biped and,
a plugin called FumeFX
I think:
this particular scene will fail to save itself (auto backup) when any kind of "modal dialog" is opened. "Viewport Configuration" and "Preference Settings" are two good examples of modal dialogs which you have to close them before you can interact with the main window again.
Submitting jobs to the backburner will also fail because 1. it is a modal dialog and 2. it tries to save before submitting.
Fortunately, I can get away with using 3dsmaxcmd.exe to submit jobs to backburner.
this particular scene will save successfully if either the biped or the plugin was deleted.
No workaround so far...
Best,
Maurice