[Windows Phone 8 Q&A, Help & Troubleshooting] GoodDayToDie:
TL;DR: I need update logs or an idea what path might be missing that breaks the 8.1 update!

Hi all, strange for me to be asking for help here but it happens...

I've finally gotten my ATIV S (SGH-T899M) into a state where I'm willing to upgrade it to WP8.1. However, I am unable to do so. I've been able to install a few updates, but the actual 8.1 update won't install. The error I get (under the "Update status" line in Settings -> Phone Update: "The update was downloaded, but couldn't be opened. (80070003)"

The error code is an HRESULT or NTSTATUS wrapper around Win32 error code 3, which is ERROR_PATH_NOT_FOUND. Note that this is distinct from FILE_NOT_FOUND (2); you only get error 3 when a part of the path to a given file (or directory) is missing. I've searched online for any information about this error with regards to updates, to no avail. It does sometimes happen with Windows Updates on PC, but the instructions for resolving that aren't easily mappable to the phone.

Phone's current status:

OS version: 8.0.10532.166 (this is the pre-8.1 version; as I said, I did get some updates)

Firmware revision: 2212.13.11.2 (this is the Samsung GDR 3 version, except modified to not overwrite WP8Diag.xap or FCRouter, etc.; see http://xdaforums.com/showthread.php?t=2537241)

Hardware revision: 15.3.1.0

Radio software: 2212.13.11.2

Radio hardware: 0.0.0.0

Bootloader version: 13.08.14.16

Chip SOC version: 1.5.32

Phone was originally branded for Telus Mobility in Canada, and still has signs of that in DeviceTargetingInfo (language is 0809 instead of 0409, for example, and MobileOperatorName is LUS-CA). It was factory SIM-unlocked and brand new when I got it. It is developer-unlocked, interop-unlocked, capability-unlocked, and has been subjected to numerous additional hacks; I've reversed a few that I can think of which could possibly impact the update (like messing with files in the System32 directory) but I probably missed a few of those.

Here's what happens when I try to update:

If Preview For Developers is not enabled, it does "Checking for updates" then says I'm up to date.

If PFD is enabled, it checks for updates and then, if I'm on WiFi, downloads the update CABs.

It then enters the "Preparing to install" step. This is very slow in some places, stalling for minutes at a time at some percentages, but eventually reaches 97%.

It then stops the update process with the error message in bold above.

It does NOT ever tell me what the update is.

It does NOT reboot, or even offer to reboot, at any point.

The "Check for updates" button re-appears and can be used to start the process over again.

So... any ideas or suggestions? I'd prefer to avoid a hard-reset if possible, simply because of the amount of time it would take to restore the phone to the condition I state I want it in, but I'll do one if nothing else works. Things I'm particularly looking for, in descending order:

1) Anybody else who has experienced this problem and found a solution (and what that solution is).

2) Any log files I could check to see what it was trying to do when the error occurred (ideally with the path that caused the error).

3) Anybody else who has encountered this problem but not gotten past it; we can compare what changes we made to our phones.

4) Anybody else who has a reasonably similarly-modified phone but has not experienced this error and was able to install 8.1 (so we can compare what we did to our phones, and I can possibly crib files off you).

Hope somebody can help! Thanks.