According to Windows Latest, the update was causing compatibility issues with drivers and software on older Macs. The support document that has been found online indicates that users who experience this issue will know, as they’ll see the message “Mac HAL Driver – machaldriver.sys: Your PC has a driver or service that isn’t ready for this version of Windows 10”.
In a comment, Microsoft said:
“Updating your Apple Boot Camp Windows Support Software drivers may allow you to update to Windows 10, version 1903. You may be able to check for updated Windows support software, in macOS, choose Apple menu > App Store, then install all available updates,”
If you want to install the Windows 10 May 2019 Update manually, we recommend you don’t until we can guarantee that the issues with drivers have been corrected. Any 2013-or newer Macs aren’t affected, and you should find the 1903 update available to you in Settings > Update & Security > Windows Update.
The company is working on a fix, and the upgrade block is expected to be lifted by the end of this month.