Ed,
I was able to reproduce your error, and it turns out the chunk size was too big. Thank you for pointing this out!
The chunk size can be updated in here in dfu.cpp.
In the sample, we reduced the chunk size by half (to 4096) just to be safe, but in my testing, I was able to reach a maximum reliable chunk size of 6900.
Please give the latest firmware a test, and let us know if it fixes your problem.
Best,
Zak