Fix supplied by speedway star over on the tenforum: https://www.tenforums.com/windows-10-news/109679-cumulative-update-kb4103721-windows-10-v1803-build-17134-48-may-8-a.html
As many have discovered, installing the KB4103721 rollup to Windows 10 v1803 may result in a black non-responsive login screen with either a rotating circle of white dots or a blue "working" circle. The only way to get your desktop back is to reboot into safe mode, whereupon the update reports it was unable to install and rolls itself back.
If you then use msconfig and disable ALL non-Microsoft services, and reapply the update, you may find that it installs perfectly. This certainly worked for me.
For interest, the non-Microsoft services I disabled were:
Apple Mobile Device Server (Apple)
ASUS Com Service (ASUS Motherboard)
BattleEye Service (ARMA 3)
Bonjour Service (Apple)
Dropbox Update Service (Dropbox)
Google Update Service (Google)
iPod Service (Apple)
Logitech Gaming Registry Service (Logitech Mouse/Headset)
NVIDIA Display Container LS (NVIDIA)
NVIDIA Telemetry Container (NVIDIA)
OpenSSH Authentication Agent (Cygwin)
Steam Client Service (Valve)
Syncovery Service (Superflexible)
I wonder if this same fix would work for you?