Inconsistent Disconnect Issue with Large Mod Count in Multiplayer

[Exodus2150]

For several months, I have been dealing with an issue where my friends could not connect to my modded Factorio server, but it was a strangely inconsistent issue. Sometimes they couldn't connect at all, while others they could connect fine, while still others it would take 5-10 minutes of attempts to get into the game; every failed attempt was met with this error:

Failed to establish Network Connection to Server.

Sometimes it'd be instant, sometimes they'd get to the password, sometimes they'd even start downloading the map before getting the error, there was no rhyme or reason.

After a week of trying to troubleshoot it to no success, we gave up and just live with it.

A strange note is that I could usually join them with no issue if I sent them the map file.

And once I moved to Germany from Miami, I knew that location wasn't the issue, because my friend from Argentina was still having the same issues, and the two that lived in Czech Republic and Croatia didn't see any noticeable improvement in connection.

Recently I had someone else try to join my world, and ran into the issue once more.

We experimented with turning our firewalls on and off, creating new firewall rules, all to no avail.

We looked at the network packets being sent and received, and noticed something odd; my computer was sending map packets, and his was receiving them, despite having been booted to the main menu.

Even making a new, fresh save, provided no result (unless the other person made it).

This had a minor cue in game: the host would see '[Player] being dropped]', despite the player never joining and already being back in the menu, and the bar would fill slowly.

We eventually tried checking to see if it was the mods, and when we played vanilla, it worked fine; so I tried trimming a large number of mods off, and, lo and behold, he was able to join with no issues.

The theory is that some combination of either poor CPU (I have the best computer with the best CPU out of all of my group), and poor internet connection (I generally had the best internet connection as well), combined with a large quantity of mods, caused players to be dropped before they could even download the map, but the the game wouldn't recognize this and sent the map anyways.

If anyone can figure out what the hell is going on, that'd be great.

Attached are the two mod lists we had before and after.

[Loewchen]

Post a small save of a session with the mods that lets you reproduce the issue.
And post complete logs of server and client of a session with the issue, see 52433.