Hardware: RLN8-410 N3MB01
Firmware: v3.0.0.148_21101146
Client:  8.8.5

I was also having this issue, so I tried to download direct from the NVR. Unfortunately, there seems to be a bug in the NVR firmware. When I attempt to 'Backup' MP4s to a USB drive, the firmware attempts to split the 3GB file into multiple parts. However, only the first part is created (even though it says 100%, success).

I was able to download successfully to USB drive by unchecking the MP4 checkbox. Doing so correctly created multiple .264 files. I then had to use ffmpeg to create .MP4 files from the .264.

Maybe this is related to the client having issues? If the NVR firmware is unable to serve MP4 files, it could be.