One file? All files? Is CS5 patched to 7.0.4?
Happens with several files. It starts working ok, and then one day, when trying to add more pages, it hangs.
Has happened for some time with some files, always with multiple (15 to 30) pages.
Patched to 7.0.4, sure.
I think I remember a bug with master pages containing 3-column frames, but I've forgotten the details and it may not be relevant here. Have you tried exporting to IDML? See Remove minor corruption by exporting
It solved spontaneously. Now it adds pages. I close this line. Txs.