I have the same unit and get the same problem once in awhile after using it for say 5-6 days . I feel the way the discharge drain enters the tank allows for build up and the internal sprayer is pointed in the wrong direction.
Using this clears it up..
this is what I do.
with the black valve open
put the wand into the toilet all the way into the tank. let it run...
go outside and turn on the internal sprayer also....let it run...
When it now looks clear close the valve for 3 1/2 minutes then open
Do this 3-4 times and I assure you no more bubbles