I had the same problem ( I think). I hovered over the viewport edge (that I could not see) till the popup said viewport them clicked, then erased. I had to do this 3 timestill it erased my viewport the undid the last erase command and put the viewport on its own layer. I have viewports matching my ps border for easy scaling reasons. I must have originally had the viewports on the same layer as my border. One was on its own layer and a different color but I did nor realize that the other two were under that one. So, I had to erase those. I gues I could done some draw order functions but this seemed to work.
Al
I had just experienced this using while editing an old drawing. I could not figure out how to delete the view port. the border around the view port was not displaying. But i looked for what layer the viewport was on and i turned the light bulb on on that layer. and the border aroung the view port appeared. I am not sure how yours is not displaying but this solved my solution.
attached are images
1) I'm hoping the OP was able to resolve his issue in the 5 years since posting. 🙂
2) The OP accounted for the layer thaw status -- ALL on/thawed.
3) This is a known isuse that has been reported frequently do a search for "HIDDEN VIEWPORT". There are a number threads on the subject which result in a lisp routine that when run will restore the VP's visibility.
Don Ireland
Engineering Design Technician
What I did with this problem is I CTRL+A in Paper space then Shift Select my required Viewport to unselect and then HIT Delete! Voila! Sh!#ts GONE!
Still an issue in 2020. I have two files, separate projects with the issue. CRTL+A and shift deselect does not work.
John Mayo
The attached has only the invisible viewports in the top left hand corner. Double click until the two viewports are seen.