New 2019 Mac - Boot camp stalls on "partitioning disk"

New iMac, two weeks old; Retina 5k, 27 inch, 2019.

I have made half a dozen attempts to install BootCamp using the May 2019 Windows 10 iso downloaded from Microsoft. Have also tried the October version.

The progress bar halts at the halfway point on BootCamp and hangs there. The "stop" button is greyed out.

I have halted the process (after several hours) by quitting Boot Camp and removing the new partitions using Disk Utility, following instructions I found elsewhere here on the help site. There are two partitions that are created each time I try to install BootCamp.

(I've managed to create Boot Camp/Windows partitions on several older macs; this is the first time I've had a problem like this.)