I did some experimenting with my system this morning. I find that in order to use WASAPI for DoP I need to have the Exclusive settings checked in Windows (Control Panel - Sound - Device - Advanced). If those are not checked, then using WASAPI for DoP gives the Playback cannot be started error. And, after checking those options, I need to reboot the computer in order to have DoP work correctly. Just checking the Exclusive options without rebooting, I still get the same error.
This is with Windows 7 and MC 20. I am not sure if it is system dependent or not. I have to check the Exclusive options in Windows and reboot the system. From then on it works fine.
Note - Exclusive mode is always checked in the MC WASAPI device settings. If I do not check the Exclusive mode in the WASAPI device settings in MC, then DoP never works. But I still have to check the Exclusive options in Windows.
Using ASIO, which is my normal setup, DoP always works, independent of the Windows setting.
My DAC is a Chord Hugo with usb, using the Chord drivers, and, of course, Bitstreaming is set to DSD in MC.