OpenVPN service dependency service failed to start (Error 1068)

Problem: I wanted to start OpenVPN as a service so that I could use a Windows Server 2003 as a router for an OpenVPN connection. The service would not start even though I set it to start automatically from the Services app in Administrative Tools, claiming error 1068: The dependency service or group failed to start.

There's a lot of people complaining about this error out there, but I could not find a better answer around than reinstalling the OpenVPN software. I am not a big fan of reinstalling without getting to the bottom of the problem - it means you haven't learned anything useful for the next time you encounter the problem.

Investigation: In the process of setting up OpenVPN, I had installed a previous version of it some time ago, and then reinstalled at a later time to get the latest version. By examining the system event log, I discovered that together with an error message (event 7001) for an unsuccessful start of the OpenVPN service, there was also an event ID 7000 stating that "The TAP-Win32 Adapter V8 service failed to start". Funny, since my TAP virtual adapter that came with the (latest) OpenVPN installation was marked V9, not v8. That nailed it:

Solution: The OpenVPN service referenced an older version of the TAP virtual adapter than the one now installed. I changed the following registry value:

HKLM\System\CurrentControlSet\Services\OpenVPNService\DependOnService

...from "TAP0801" to "TAP0901", rebooted and the OpenVPN service had already connected by the time I was logged in.