I frequently download oscilloscope curve data using pyvisa. Twice I did run into trouble:

1. For downloading the curve the oscilloscope must be in the stop state (thats why I always use single shot to aquire the data). In the run state downloading curve data always fails.
2. Damaged USB port in the oscilloscope. Using the same device over a network cable did function without any issues.

So make shure the oscilloscope is not acquiring new data while doing the download and try using a network cable instead of the USB-Port.