I have solved this with the help of Stratasys and GoEngineer (kudos to GoEngineer!)
There is scant support info out there on these printers so I plan to post when I discover something useful or solve a problem.
Behind the filament drive wheel in the print head, on the left (support filament) side, there is an optical sensor looking at the drive wheel.
When the machine begins to load material (spooling out filament from the canister), if the sensor is obstructed, it will stop feeding after about 5-10 seconds, and report that there is an obstruction.
In my case, I had a bad filament load and a small piece of filament broke off and fell behind that drive wheel. Removing that solved the problem.
One clue that it was not really a filament obstruction was that it did the same thing on both sides. But of course I did run a piece of filament all the way up to the head manually just to be sure.