cannot upgrade

[Jerreke]

hi,
while upgrade my plesk 7.6.1 to 8.1 I got following error:

The upgrade patch cannot be installed by the Windows Installer service because the program to be upgraded may be missing, or the upgrade patch may update a different version of the program. Verify that the program to be upgraded exists on your computer and that you have the correct upgrade patch.

I've also tried upgrading from 7.6.0 to 7.6.1, but same error

Someone got a clue?