I had the same problem. I did the IDML trick and it worked by finding and linking the file which was not previously available. I was then able to UNLINK.

But a faster way is to relink to any text file and then UNLINK. Since these are old links and you're not going to use them anyway, it doesn't matter to what they're linked. Getting access to that UNLINK is the goal.

Weird little problem in any event.