I can't really say, if it is a bug or not (sorry, if not). I'm using portableapps in a folder I:\vm\shared\Portables, and it is also used by virtual machines (Win7, win10), which are mount this directory with Z: in my virtual machines.
Now, to have the same driveletter on localhost, I mounted on it Z: => \\localhost\I$\vm\shared to have the same drive-letter on my localhost, than in the virtual-machines. But all browsers I start in localhost under Z: does not display anything (firefox, opera, chrome), browser is running, but the window is only white (menus are visible). Only kmeleon is working. So:
I:\vm\shared\Portables\PortableApps\FirefoxPortable => no error on localhost
Z:\Portables\PortableApps\FirefoxPortable => no error on vitrual machine
Z:\Portables\PortableApps\FirefoxPortable => ERROR on localhost
Same with Opera, Chrome etc. All other Programs (instead of modern browsers) are running perfectly.