Thanks for the debug output. It does confirm that the tftp request was sent to the server and that a response was received. This is good to know. I see in the debug output that the server responds with a block size of 8192 and the switch is sending 8192 length packets. Given the path that this data takes, and given the description in a previous response that part of the path may be over vpn, I wonder if the issue is about packet size and possible need to fragment. Is it possible to configure the server to specify a smaller block size?
Thanks for the update letting us know that the problem is solved. I am glad that my suggestion pointed you to the solution.
I believe that the explanation to your follow up question is that while it is correct that 3850 will default to block size of 512 that the block size for any particular transfer is negotiated. And if the server negotiates 8192 then the 3850 will use that block size.