?
Of course, it's possible that NuGet 3.4 works with your proxy servers differently. I don't know.
But in either case, it's most definitely a bug in the 3.4 client; a "nuget push" is a very simple HTTP POST or PUT with a body containing a single file with a multi-part encoding, so if you are getting that error, then no file is being sent (unlikely multiple files). You can use cURL or any other tool to send file to ProGet like that.
There is nothing we can change on the ProGet's end to work-around this bug, so please report this to the NuGet team. You should use Fiddler or Wireshark to capture the output from NuGet, and find precisely what requests it is struggling with, so you can share it with them.