My party members always crashes (Debugged)

Issue Summary:
I've tried playing with my friends multiple times, but anyone who joins my party eventually crashes.

Debugging Attempts:
A friend and I analyzed the issue thoroughly, testing various theories and examining crash log files.

TL;DR:
Party members crash whenever they move far away from me. If we stay close together, there are no problems. We tested this multiple times with consistent results.


Test Cases:

I'm Player A, with friends B and C.
A-B play: B crashes when moving far away.
A-C play: C crashes when moving far away.
B-C play: No issues at all.


What I Tried:

Played with both keyboard (WASD) and controller.
Verified game files multiple times.
Connected only via the Steam client (plan to test with the launcher soon).
Tested in both Atlas maps and Acts.
Ran tests under different conditions:
Moving far apart without using skills.
Staying far apart without movement (checkpoint teleportation).
EDIT: Staying away without killing any mobs doesn't crash.

Notes:
From my perspective as a software engineer, the log files did not reveal any obvious issues. I can provide log files if needed.

EDIT: Added to what I tried

I`m player B and I approve this message.

Happening to my brother and I today, he is the one who gets disconnected. Were either one of you using an item with, "Allies in your presence" on it? I changed out my scepter (and it has different "allies in presence" mods) and that is the only thing different from yesterday.

I'm having similar issues. No problems with my 5-man play group before, but after I respecced to a fire build my friends frequently crash if more than about a screen away from me in maps.

Never happened before I swapped to a solar orb / flame blast setup.

New spells used in total are, in order of frequency of my use:

Flame Blase
Solar Orb
Incinerate
Ember Fusillade

Something about this new setup seems to be the issue.

I'm also using grim feast w/clarity but that is not new.

Update!

After testing, it seems to be Fiery Death Support. Whenever it is socketed and I proc a corpse explosion, distant allies crash, 100% reliably.

I appear to have the same problem with my fire build.

Fireball/Flame Wall/Solar Orb

If I join friends map and I am not on same screen and I just use an ability their game crashes.