This is a problem with xdg-open (which is used by Do to launch URIs) and not Do.
$ xdg-open "smb://smbserver/www/"
Error showing url: The specified location is not mounted
$ nautilus "smb://smbserver/www/" -> works
Digging through the Nautilus source I can see they check if it is mounted, and do a mount if it is not. It seems like maybe xdg-open is checking if it is mounted before calling the appropriate application (Nautilus). Please file a bug report with their maintainers.
This is a problem with xdg-open (which is used by Do to launch URIs) and not Do.
$ xdg-open "smb:// smbserver/ www/"
Error showing url: The specified location is not mounted
$ nautilus "smb:// smbserver/ www/" -> works
Digging through the Nautilus source I can see they check if it is mounted, and do a mount if it is not. It seems like maybe xdg-open is checking if it is mounted before calling the appropriate application (Nautilus). Please file a bug report with their maintainers.