What about Windows default audio drivers, have they all been removed except those that belong to the Tascam.
 
 

what do you mean? the drivers for the unit? They are the ones off the site.

Remove all Windows audio devices and Drivers
 
Device Manager > Audio inputs and outputs, (Only Octa-Capture is listed)
 
If your computer is only used for your DAW, then all other audio drivers should not be present except those belonging to the Tascam device.
 
With my Computer only used for Sonar, T-Racks and my plugins. I have no other audio device drivers but those belonging to Octa-Capture and I blocked all updates from Windows 10