After some digging, I found that Xtreme mod is prone to not responding to hashset requests somehow. By reviewing the codes, the 'Xman improved socket queuing' feature along with other socket related improvements seem suspicious.
Though I am still struggling figure out which part of the codes is to blame, it would be great if someone familiar with the mod's codebase can track it down and solve it. After all, Xtreme mod is still very popular.
Edit: The error log is in 'Log' tab, look like "MD4 HashSet not present while veryfing part %u for file %s". It happens when most probably the first connected client is Xtreme mod client, and it failed to response the hashset request, while the following connected clients started to uploading to us. When a part's finished, we found that the hashset is not available yet.
It will last for quite some time, and getting even longer if UDP re-ask always succeeded (since no TCP connections for another round).
This post has been edited by Enig123: 22 April 2018 - 10:46 PM