One guess is that the version of Arial Narrow you have installed now somehow differs from what you had before. For example, maybe before, Arial Narrow was not treated as a subset of Arial, in the Narrow weight, it was simply a separate font named “Arial Narrow”. Then later it got organized so that black, narrow, bold, italic, etc. all became part of a single Arial family.
I'm not sure if it has to be literally the exact same font file to make ID happy, or if you just need a font installed with the same internal font name (which can differ from the font's actual file name, you need a font editor to change the internal name).
You might try importing a copy of Arial Narrow from a different PC, one that didn't go through the upgrade.
For reference, my version of arial narrow is named ARIALN.TTF and is listed under Arial in my font menu, works fine in my copy of CS6.
Filesize is 169 KB (173,936 bytes).