FWIW, I am experiencing the same symptoms as described by the previous posters.
I can ‘see’ my DAC in the dropdown / selector, but nothing can playback without rebooting volumio, if I power off/on the external DAC.
I am running Volumio 1.5 on a CuBox-i4Pro with a USB DAC (NAD D 1050).
Otherwise, everything works great.