Just to add, I did wonder whether obtaining a more recent version of the BootCamp WindowsSupport package might contain a newer version of MacHALDriver.sys that supports 1903, and sure enough it does.I just looked into this myself. Just moved from 1803 to 1809, as Microsoft’s track record for these upgrades isn’t great.
Performed an upgrade to 1903 on a test system and hit a BSOD on login; WDF Violation.
If you have an older version of the BootCamp drivers installed that supported Windows 7 (I’m using a MacPro5,1), then the root cause is apparently MacHALDriver.sys located in C:/Windows/system32/drivers.
This file needs to be moved or renamed for 1903 to run properly, but the side effect is the BootCamp software will stop running, killing your use of the function keys (I have an ACD, so this is important), as well as being able to tell the system to reboot into macOS conveniently.
If you go here, this chap has developed software to speak to the ACD if you’re just using a PC, so this should be a workaround.