Recently a font installation on my Windows 10 system failed. This seemed to have messed up the system font, as on the next reboot, filenames and most UI menus appeared in hieroglyphs.
I followed the instructions on Windows 8.1 missing font files after restart: I replaced the fonts in the C:\Windows\Fonts directory with the default ones and deleted the registry key
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Fonts. As a result, the next time I wanted to boot, I couldn't even log in anymore. The login screen never appeared, instead I saw a screen with the loading spinner, that flickered 2-3 times per second. Safe mode in all its variants makes no difference.
From the startup recovery command line, I restored the backed-up registry key and also tried to adopt a friend's key and fonts directory 1:1, both to no avail. Now, I'm faced with an unusable Windows 10 System, all a result of installing a broken font. Unfortunately, I don't have restore points, so I would have to reinstall my entire Windows if I can't fix this.
Is there a way to repair the broken login screen? Unfortunately I don't even see a way to run PowerShell from the recovery CMD (to follow this answer which seems very promising). At least I managed to back up my data from that CMD.