Unable to Open Audio Playback Device!

Sonar 4.04 Studio, Win XP Pro

This involves projects (mostly MIDI, but using DXI and VST synths) (a little audio as well) created using my Echo Indigo I/O as the audio device.

That card is currently not working. (See other post about it.) Therefore, now using my Edirol UA-20 (WDM mode).

With some projects, when I click on the playback button, I get the error message:

"!Unable to open audio playback device. Device may not support the current audio project's audio format or may be in use."

The song doesn't play. Not just one, several of them, all with DxIs. On the contrary, if I play back a MIDI-only project that uses my system-wide synth (in my case, Yamaha SYXG50), not the dxi or vst plugin type, it will play back fine through the UA-20.

The device is not in use by another program. To avoid such conflicts, I set the default audio device in Control Panel to the laptop's built-in sound card, not the UA-20. There are no other audio or MIDI programs running, other than Sonar.

The audio format--I haven't done anything fancy with that. I don't know that one specifies audio format for a plugin synth. If so, I doubt anything different than standard 44.1 16 bit stereo.

What could be causing this? How do I fix it?

Thank you.