Windows 11 not activating after mainboard replacement, troubleshooter doesn't run, can't get support

I work from home with my PC running Windows 11 Pro 25H2. My Windows license has a long lineage, starting with a retail Windows 8 DVD, which was successively updated for free to Windows 8.1, 10, and 11, activating it with a digital license automatically issued by Microsoft at every update and renewed without issue at every Windows reinstall.

So, I don't have a Windows 11 product key, and since the original retail purchase was over a decade ago, I don't have the Windows 8 DVD or its key anymore either (not that I think that it would work anyway).

During all those years, my PC had several hardware changes that affected the activation, but back in those days you could easily talk to a Microsoft support representative, and they always solved my problem. Now, however, Microsoft has made that extremely difficult. More about this later on.

My mainboard just died a few days ago, and I was forced to replace it. I did not reinstall Windows, I'm still using the same old boot drive with the same Windows install. I knew I would lose the Windows activation, but I counted on that being easy to resolve, as it was the previous times. Was I wrong!

I searched what I should do and found that I had to run the activation troubleshooter on the Settings -> System -> Activation window and inform that my hardware had changed. However, when I click on the Troubleshoot button, nothing happens, other than the mouse cursor briefly blinking. THE TROUBLESHOOTER WINDOW DOES NOT APPEAR AND IT DOES NOT RUN. I ran sfc /scannow, rebooted, and tried again. Nothing changed.

I tried to contact Microsoft's support, but after walking in circles through a lot of knowledge base articles that the site force-fed me and that obviously didn't help, I finally found an actual contact link. It asked me for the type of issue, and I selected "Windows activation". After another truckload of useless KB articles, it asked me to inform my installation ID (not the product key), which, according to that page, should appear on the Activation window. The window does not show the installation ID.

I searched again and found that I could obtain the installation ID with the command slmgr /dtl. Indeed, it showed the ID — in a separate pop-up window that doesn't allow copying the text. It's a very long sequence of digits (63, I think) that I had to manually and carefully type into the form. I had a few false starts (it's easy to mistype when it's 63 seemingly random digits), but the form warned me that the ID was invalid and I retyped it until I got it right. Then I got a response that said that my installation could not be validated and I could close the window. No contact link, nothing more. Dead end. Microsoft refused to even hear me!

So, the recommended solution doesn't work and Microsoft won't even let me contact them and explain my problem. I don't want to buy a new Windows license, not only because of the expense, but also because it wouldn't be fair. After all, until a few days ago I was a legitimate user with a fully licensed and activated Windows install, according to Microsoft's own terms. In the meantime, I'm writing this with that ugly watermark on my monitor...

What can I do? Thank you very much in advance for any help.