If you still have this issue, I have posted a similar issue, and here is the update, hopefully it is the same solution:
run this commandon the windows client machine:
netsh interface tcp set global autotuninglevel=disabled
Honestly id stick with the windows server for now. The new version is having "growing pains". i Think the last stable version was 6.35.1?
Appreciate it.Follow tutorials on this link,they are very useful
http://mikrotik-q.blogspot.com/2015/05/ ... outer.html