mshta.exe can't run html application from a non-regular network drive

Recently, I had problems installing vcredist from a network drive. It was because the exe file had to be accessible by the Windows Installer service. To solve that I mapped the drive as NT AUTHORITYSYSTEM to make it visible to all users, see http://superuser.com/a/1103115/233620

Later I found that IBM DB2 installer launchpad for Windows hangs or dies in such setup. The launchpad is based on an hta application. I was able to reproduce the problem with a "hello world" hta application.

C:> mshta.exe \okdistrhwdistr-rounmountablemischello.hta
# ok
C:> net use y: \okdistrhwdistr-rounmountable /PERSISTENT:yes
C:> mshta.exe Y:mischello.hta
# ok
# mapped \okdistrhwdistr-rounmountable to Z: as NT AUTHORITYSYSTEM
C:> mshta.exe Z:mischello.hta
# mshta process dies instantly