Cannot open files on network

When I try to open a PDF from the network, I get an error message:  "There was an error opening this document.  This file cannot be found."

I have no problem opening PDFs from within Acrobat.  (The problem exists only when trying to open from an Explorer window or another program.)

I cannot think of any change that would have caused this to start.

I have tried Repair.  I have tried uninstalling, using Acrobat's "cleanup" tool, and reinstalling --- the problem persists.

This is significantly impairing my ability to work!

Any idea what is causing this?

Thanks.

Micah