Are you saying you are getting an error message when you switch to Trip B that says, "PAUSED"? On my '14 when I go to either Trip A or B I have the opportunity reset the trip by pushing the Down button or to temporarily PAUSE the meter by pushing the Up button. When I push the Up button the option changes "PAUSE" to "RESUME" and the reading on the trip meter stops recording at whatever mileage was on it when I pressed "PAUSE." Try this: go to Trip B , select "RESET" by pushing the Down button. Turn off the ignition and give the Spyder a few minutes, 10 or so, to reset everything. Turn on the ignition again, and Trip B should show the first time you press the Mode button and it should read 0.0. Ride around the block and see if Trip B starts to register mileage. If it registers you have successfully reset it. Now push the Mode button until Trip B comes up and the mileage should be as recorded and you have the option to push the Down button and Reset it again or push the Up button and and "PAUSE" should change to "RESUME" and the mileage should freeze at whatever you had traveled. Ride on a bit and Trip B should not change. If it doesn't change, stop again, push the Mode button until you come to the window where you can "RESUME" the trip, Push the Up button once, drive on again and see if the trip meter resumes reading mileage as you roll down the road. If it does, the system is operating as it's supposed to. If it doesn't resume, it's time to see the dealer.
Oh, it's immaterial which Trip meter you use to measure mileage per tankful. FWIW, I use Trip B to record daily or tour distances and use A for my tank mileage. My Zumo 590 also backs up my A distance for fuel refills.