There are two reasons you might have ink key generation problems with a QMDI in Raster Blaster 3.7.8 

First, check to see that you are running the correct version of the output plugin for RasterBlaster 3.7.8.  We had an issue that was resolved in a new output plugin.

You can get the latest one here.

You also may have a configuration problem, which you can read about here:

QMDI ink key configuration.