Note that exclusive output - like all output plugins of the sort - doesn't perform any resampling of its own; if your device isn't agreeing with the sample rate of your audio files, errors happen. This is currently the intended behavior.Thanks Peter for your work on getting this right but I would encourage you NOT to engage some auto-resampling routine to Wasapi exclusive. As you described, passing the file rate straight through to the device is intended behavior. Users with devices that are not compatible with the native sample rates of their files already have several options to handle this via Foobar DSP resampling plugins. IMO, why add an extra layer of handling to something that is proving tricky enough as-is to fix.
I'll think of some solution for this for post-1.6.7, perhaps negotiate some optimal rate to resample to.