tl;dr version: Make sure you are not trying to open a file marked in the file system as Read-Only in Read/Write mode.
I have come across this error in my travels trying to read in an XML file. I have found that in some circumstances (detailed below) this error would be generated for a file even though the path and file name are correct.
File details:
When trying to read the file programmatically, the following behavior was observed while running the exact same code:
In order to fix this, I had to change the method call from the default (Opening as RW) to opening the file as RO. Once I made that one change, it stopped throwing an error.