It seems indeed that there is a bug related to thieves guild leadership:
If the player completes the main story arc before doing the side and improvement quests for Vex and Delvin, Brynjolf may not interact with the player and instead repeat that he is busy and that he will talk to you later. Consequently, becoming the Guildmaster will be rendered impossible. This may be a part of a bug where Karliah disappears and does not appear in either the Nightingale Hall or the Twilight Sepulcher. She does, however, return to the Snow Veil Sanctum where you first encounter her.
Possible Fix:
Run outside when finishing your first interaction with Brynjolf. You need to run outside as quick as possible by going to the ladder and exiting. You should see the quest update pop up reading: "Started Under New Management". Afterward, go back inside and proceed to Brynjolf. To talk to Brynjolf, let him go to the middle of the Cistern first, and wait. He then should start talking to you, finishing the quest. If this does not resolve the issue, it may be worth it to try attacking them, paying the reparations, and trying again.
Another possible fix is to get the quest about fixing debt from the guy at the Riften stables. The player will be told to talk to Sapphire. Instead, Sapphire should be sitting on the bridge just inside Riften and when approached will say "I don't have any business with you." or "We'll talk after the ceremony." Attack her 3 times, and this should cause her to start attacking the player notifying them that "Under New Management" has been completed and that the player should talk to Brynjolf. Upon entering The Ragged Flagon, the player will be required to pay 1000 gold to Vex as reparations for killing a fellow Thieves Guild member. After doing so, speak with Brynjolf and the questline SHOULD be completed.
As a last resort, you can use the console to fix this issue:
setStage TGLeadership 0
How to fix (2013 update)
I've tried the bugged saved game file I initially posted in my question with the current latest versions of the official patch, Unofficial Skyrim Patch (USKP) and Skyrim Script Extender (SKSE), and the saved game is now Aspect of Peryite-free.
I've configured SKSE to use its ClearInvalidRegistrations
console command on every saved game load, which removes invalid scripts left running by uninstalled mods. This feature was introduced in v1.6.7 of SKSE:
add console command ClearInvalidRegistrations
to remove invalid
OnUpdate() registrations
This prevents orphaned OnUpdate() events and
the resulting bloated/broken saves when removing certain mods. When
applied to an already bloated save, it will stop growing further and
instead shrink over time as the game processes all queued events. This
may take hours depending on the amount of bloat.
To execute automatically after each reload, add this to \Data\SKSE\skse.ini
:
[General]
ClearInvalidRegistrations=1
According* to a developer of the Unofficial Skyrim Patch (USKP), the underlying cause is because DA13 ('The Only Cure' quest) is never stopped.
A possible fix* for this:
In the meantime, load an affected save, then type stopquest DA13
into the console. That ought to put an end to that. Plus it’ll free up some unnecessarily leftover alias memory.
BTW, don’t use the console command on a save not bothered by it. Even though it may appear benign, console commands can have negative consequences you don’t know about.
If this doesn't work or if you experience issues like: NPCs not responding properly to conversation (not reply to you when you choose a certain conversation path - leaving you stuck), merchants not opening the buy/sell window, horses that can't be ridden, and other interaction issues, then the save game is most likely corrupted. Try the SKSE ClearInvalidRegistrations
console command fix I've posted above, instead.
To prevent this from happening in the PC, get the latest version of the Unofficial Skyrim Patch. This has been fixed since version 1.1 of the USKP:
"The Only Cure" (DA13) was never stopped which would lead to the Aspect of Peryite randomly initiating conversation and being unable to exit.
* This info is from the old USKP bug tracker page at 16bugs.com, which is now not viewable. The new USKP bug tracker report page of this bug is at afkmods.iguanadons.net.
Best Answer
How to fix it?
Based on the information below, the question is less a matter of "how to fix it" and more a matter of "if it can be fixed (right now)."
You currently have a few of options, and a couple aren't ideal but may be the only recourse:
While a few users have fixed some of their issues by ensuring load order has placed the Unofficial Patch first (top), many others report CDT (crash to desktop) when getting near a populated area (Riften is one of the common cities CDT is reported to happen while using the Unofficial Patch).
Why this is happening
On January 9, 2017, Bethesda released an official patch for Skyrim SE and the game version is now 1.3 (1.05 on PS4, and I believe same versioning structure for XBox One). However, PC and PS4 were the only systems to get this update.
From the Bethesda site, the latest devnotes state:
UESSP shows the latest version of the patch is 1.3 to match what's available on PC and because the mods available to consoles are PC ports. UESSP states two clauses regarding the Unofficial Patch:
From this, it seems part of why you're having this issue is because the Unofficial Patch addresses lingering issues of the official patch you do not have access to - meaning the unofficial patch has altered code that could cause bugs with official version 1.2 on your X BOX but not cause a problem with official 1.3 on PC.
Of note, many forums have flooded with XBox One mod and patch problems over both PC and PS4 in the past month (around the time the Unofficial Patch 1.3 was released). I can only speculate correlation, however.
This makes me angry.
A post on GameFAQs pointed out the Unofficial Patch has seen a growing number of problems, but that it isn't really a single party fault as much as it is a circumstantial one:
Great...what else can go wrong?
A comprehensive list of bugs is under constant review.