FYI to future people with this problem:
After relentlessly just hammering taking the quest over and over hoping for a miracle, only to get d/c’d, then log back in and dropping to my death in Arathi, I abandoned the quest to quickly take me back to Bastion, while falling. I logged completely off. One other thing I did based on researching other threads for this issue was going into my WoW folder by clicking on the cog wheel on Battle.net and selecting Show in Explorer. Then I renamed my Cache to CacheOld, Interface to InterfaceOld, and WTF to WTFOld. I also removed all quests, and all addons (not just disabled, but uninstalled), and even restarted the PC for good measure. With fingers crossed and raised hope, I took ‘He Was My Student’…and d/c’d again.
However!!!..THAT last time I actually saw my character appear in the Stormwind before getting d/c’d, which hadn’t happened before. So I logged back in, began falling, abandoned the quest, show back up in Bastion. Normally when I did this routine I waited for the PVP flag to drop, but this time being excited that I saw my character in Stormwind, I did not wait and clicked on the Soul Mirror, and I was able to finally finish this quest.
I hope a combination of that stuff helps someone else out, going forward. What an ordeal.
After happening to me a dozen times or so, what I did was drop the quest, leave the area where the quest was and fly around Bastion for 15 min or so, then log off and re-log on, then fly back to the location and pick up the quest. Worked first try.
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Still happening. Ticket submitted. No response at this time
Tan
Just read your solution and tried it. It worked. Thanks alot 
I kept having this problem then accidentally found a way around it. Where you click to enter the memory there is a “wall” in front of Uther. I went around the other side of the wall (carefully) and clicked the teleport spot via the wall itself. THAT is what landed me inside the room for the scenario. Otherwise, I would be outside the door and was disconnected time and time again. I hope this helps someone!
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Still bugged 3-8-2024. Abandoning ALL quests, disabling all addons, and /reload worked for me after 3 falls.
Yep still broken tried it today and no addons at all no pvp on and it jumps to stormwind and instantly disconnects.
Ok third try i abandoned quest then got it again and it instantly took me to stormwind and didnt disconnect.
For those who don’t know: Cancelling the quest in the air will teleport you back to bastion. Took me 2 falls and run backs to figure that out
On my 3rd, and successful, attempt after I abandoned the quest I typed /reload and reset all instances by right clicking on my portrait. I let all NPC dialogue finish before entering the instance. I do not run add ons and was still flagged for pvp
And the bug claims another victim - I’ve tried everything listed here with no success, over 10 long falls now. I have a feeling there’s a timing issue or similar on the server side such that 90+% of the time it crashes, and the reports above are just “that’s the one time the server happened to work properly” rather than an actual workaround. I ended up filing a support ticket explicitly asking to have the quest flagged as completed, seeing as I don’t really have any other options.
Just imagine if Blizzard had taken all the effort they put into Plunderstorm and used it to fix bugs like this instead…
To follow up, I did get the ticket resolved and the quest set to “ready to turn in”, on the Tuesday evening after sending it in Friday night - not too bad of a turnaround time, I guess? The GM who responded confirmed that they’re aware of the bug and that the “developers are working to resolve the underlying issue”, but that there “is currently no workaround”.
So it looks like the solution if you run into this is “file a ticket and do other stuff until you get a response”.
Here is what worked for me: I abandoned all quests I had, including He was my student, then re-accepted and started the quest. Everything worked from then on.
found a fix from blizz help center, go to options>system>network & uncheck all 3 boxes ipv6,optimizespeed and combat loging.
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Doing this, abandoning the quest, then getting the quest again, and clicking the wall from the back allowed me to do this.
Just happened to me too so I came here looking for answers.
On my third attempt I quickly alt-tabbed immediately after clicking the soul mirror. When I tabbed back after a few seconds I was in the scenario with no D/C.
Hope this is a work around and wasn’t just luck.
Edit: I just realised as I tabbed back into the game that I wasn’t able to attack Arthas and essentially couldn’t progress. I left the scenario and turned pvp mode on then clicked the soul mirror and alt tabbed again. Upon tabbing back into the game this time Arthus was hostile and I was able to complete the quest 
Just had this bug. No manner of attempts got me further, than clicking the memory from the OPPOSITE side. No clue why this worked.