opening dwg in Windows 10

Hi, new user here. I'm using Windows 10, downloaded a .dwg file that was edited in an older AutoCAD, trying to open the file I get this message:
"Cannot open the file ... Please check its existence and permissions."

I'm posting here and not in troubleshooting because I have two issues with this:
1. I want to open this file
2. I think this error is not descriptive of the actual problem. The same behaviour occurs when I'm running LibreCAD as Administrator, and the file is most certainly extant and a valid dwg format.

I'm afraid can't provide the file that isn't opening. I tried a sample dwg, which opened just fine. The permissions settings and other properties shown by Windows Explorer's context menu's 'Properties' appeared to be identical, besides the file names and modified dates.

Any ideas?

Edit: I noticed that another CAD app managed to open the file, and issued a warning that a .png image file reference does not exist. I saved a dummy .png file there - no change, still get the above error. Confirmed that the warning is not issued by the other CAD app now that the dummy file is in place.

see http://forum.librecad.org/Open-dwg-error-Visual-c-tp5717162p5717164.html