This is usually an indication for a corrupt PDF file. Does this happen with just one file, or files from one specific source? That would point towards a corrupt file. If it happens with all PDF files, then you are either dealign with a bug in Acrobat (even though I am not familiar with any potential bug that would have this symptom), or with a corrupt installation.
When you hold own the Ctrl key when you click on the "OK" button on this dialog, you will get a more technical message. What does it say?
It doesn't seem to be happening with any particular file, it's multiple files and seems to be somewhat random. I'm really not thinking it's related to a corrupt file. The only common thing I have noticed is that it seems to be larger files that are doing this.
I will get back to you with the technical details.