Cannot activate Windows 10

I bought a Windows 7 Pro licence a couple of years ago. The installation got activated, of course. I've been working on the machine for some years. Of course, the licence has been used only on this one machine.

During the free upgrade offer, I did an inplace upgrade to Windows 10 Pro. The installation was activated, too. That's how it's supposed to be done. Once activated this way, it is valid to do a clean install of Windows 10 on the same machine.

The problem: After the clean Windows 10 reinstallation (version 10.0.14393) on the same machine without any hardware changed, it can not be activated. I'm getting the message that the activation servers cannot be reached. This can be read there for several days. I have a direct internet connection without a proxy. I've ensured that the firewall is not blocking anything related. In the event log, I can find the error number 0x80070426: The service cannot be started. I'm an experienced developer. I was not able to find out which service. All deactivated services are unrelated. The activation trouble shooter does not work because of the same error 0x80070426.

There is the slmgr.vb script to manage licences. Using "slmgr /ato" returns the same error. Starting the script (to be exact: cscript.exe executing the vb script) from within WinDbg shows a different behavior: Now WmiPrvSE is started, it connects to the internet and finally tells me that the product key is blocked. I do not know why. I think I did everything right. The only "helpful" answer I was able to find, was doing the whole process again, starting with the installation of Windows 7. For sure, I will not do this as it will take me several days again (yes, it does). Other reports of the problem were different as to at least one of the preconditions weren't met.

I'm also aware of this: How to troubleshoot Product Activation in Windows 10

My understanding is that you installed Windows 10 as an upgrade and then tried to install Windows 10 on the same machine as a clean install.

If you did not deactivate the key or deactivate the installation before you did the clean install it will still be registered as in use. (Normally this shouldn't be a problem as the machine's fingerprint would be the same.)

In this situation, you'd need to call to get the key activated for your current installation. (This will also deactivate it for any previous installations.)