For the last two years, I've been using Umbraco CMS for a few sites. It's a great content management system, giving you much freedom and easy extensibility while still maintaining a reasonably learning curve. It's probably not suited for typical 1-click-end-user-install scenarios, but it integrates really well in a .Net / Microsoft technologies stack with source control and continuous integration.
Today, however, I wanted to log in to the backend of one site to change something. It didn't work. The login was successful, but then I only got 401 responses from the server, rendering only a blank, white screen instead of the expected Backoffice. Another site, the same problem. Every Umbraco site on that server showed the same symptoms. The front ends were rendering just fine, with everything working as expected, but I could not access the administration section of any of them. I tried quite a few things, from different browsers to different devices to different ISPs, but the problem persisted. Search results for this issue were not particularly helpful, most being outdated and the bugs been fixed since a few versions ago. The logs were no sources of wisdom either, just showing a lot of successful login attempts without follow up actions or failures. Nothing had been changed on the sites, so I didn't really suspect it to be an issue with Umbraco anyway, and I went on to check the server.
The Let's Encrypt extension did update two certificates in IIS that day, but that's happened before and shouldn't cause any problems. By chance, while being remote on the server, I noticed that the servers time was off. A good two hours. I fixed the time, tried to log in and - success!
Maybe I can save you some time digging through Umbraco issues when all you should do is check your servers time.
Happy logging in!