So for a long time Remote Desktop (RDP) into one of my Windows servers worked just fine. Until it didn't.
I have no idea what happened and what broke it. But one day, trying to RDP into a Window Server runing Windows Server 2019 Build 17763.2366, I just got a black screen. After a few minutes, RDP would announce: An Internal Error Occurred. Yay for helpful messages.
First, I googled the symptoms, and found various reports of the same symptoms. Some "solutions" were suggested, which I dutifully tried. But of course, none of them worked. And nobody really knew why this happened.
So I used Remote KVM to take a closer look. Visiting Event Viewer > Application and Services Logs > Microsoft > Windows > RemoteDesktopServices-RdpCoreTS > Operational and I eventually found this:
Connection doesn't support logon error redirector' in CUMRDPConnection::GetLogonErrorRedirector at ...
Well, I didn't know what to make of that, but with even more googling I found that by editing the registry Computer\HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Terminal Server Client and adding a DWORD value UseURCP = 0, everything starts to work again. Magic!
So, how did it break? And why do I have to add registry settings to fix it? Nobody knows... although I suspect a Windows Update messed things up, because that server was otherwise untouched.