Photo Downloader freezes after few pictures

Each time I try to download pictures to Bridge using Photo Downloader, it freezes after the first 25 - 35 files. Pressing 'stop' has no effect - button just greys out. I can also see there is no CPU or memory being consumed. 

This is a brand new Bridge install on a brand new PC:

Windows 10 Pro, 64GB RAM, AMD Ryzen 3960X CPU, NVIDIA GeForce GTX 1650 Graphics card

Bridge V10.1.1.166 - clean install, no changes to preferences made

Option to convert to DNG selected

Camera is Nikon D850, downloading from SD card via USB3 Card Reader

I have tried connecting the card reader directly to PC (rather than over USB hub), and also tried going directly to local drive rather than to Synology NAS. Also tried restarting PC.