Just in case anyone else does a search and comes across this thread sometime in the future like me:
I had the same problem, and the solutions in this forum didn't work until I set layer 0 to be the current layer, then I was able to select the viewport.
Before I tried that, I had all of my layers on and thawed and unlocked, but some other layer was current, and no matter how many times I selected everything, there were no viewports in the selection set.
So it seems to me that the solution would be to turn on, thaw, unlock, and make current layer 0.