Unable to start solo duty in "The Ultimate Weapon"

[Posting on behalf of a sprout friend, who's not on the forums.]

They started the MSQ quest The Ultimate Weapon after the 6.1 maintenance. After reaching the step "Confront Lahabrea on the Porta Decumana.", you are supposed to fight Lahabrea in a solo duty.

Starting this solo duty is impossible. After the cutscene plays, the duty does not start, and "A system error occurred during event movement." is printed in chat. This error implies this is a serverside issue, and not an issue with client integrity.

Abandoning the quest is not possible - therefore this softlocks the MSQ. They submitted a bug report using the in-game Support Desk but after a few days no reply has come.

Things they have tried:
- Restarting game
- Restarting PC
- Exiting and entering the area
- Waiting for the cutscene to play out in full
- Using the World Visit System to attempt the duty on another world
- Abandoning the quest (not available)

Date & Time: After Patch 6.1
Frequency: All
World name: Gilgamesh
Character name: Nyaku Astaze
Retainer name: N/A
NPC name: Lahabrea
Monster name: N/A
Class/Level: PLD, 50
Party or solo: Solo
In-game time: N/A
Area and coordinates: The Porta Decumana (world area reached by using the passage next to The Praetorium's entrance - not the trial!)
Housing: N/A
Steps:
1. Accept the quest "The Ultimate Weapon" after Patch 6.1
2. Reach the quest step "Confront Lahabrea on the Porta Decumana."
3. Speak with Lahabrea to start the solo duty
4. Notice how the solo duty does not start

Playing with two sprouts and same exact thing is happening to both of them also. They tried Saturday and Sunday with no luck. They are on Diabolos server.

This is also happening to my sprout. They cannot progress and it errors everytime he starts his the quest for the solo duty..

This appears to have also been reported in the Japanese thread below.
It has been accepted and is under review presently.

https://forum.square-enix.com/ffxiv/threads/460457

This error seems to have been related to instance server availability when Stormblood released.

Update!

My sprout just updated his gear to get the max ilevel and it seems to have let him in and hes finally progressing.

They should still be specific when it errors and tell him to update his gear though I guess. But it seems to have fixed the bug.

Update, this has been accepted into "confirmed bug, plan to fix" in the following duplicate thread below.

https://forum.square-enix.com/ffxiv/threads/460719

Appears average item level 42 or above is necessary to proceed (information from the support desk), but this requirement is not communicated properly to the player.