I too am having this issue on all my sites with the newest version of WordPress. I have tried every fix I can find or think of. Disabled all plugins…modding the htaccess file…modding the functions.php file…ensuring I am running php 5.6….everything. No matter what, the upload starts, then says connection reset. In the past, I would only have this issue extremely rarely. Now, it is happening every time. My host cannot find any issues and have maxed out the resources. The only factor left is the newest version of WordPress itself.
Ideas?