1. Used DDU in Safe Mode to uninstall Nvidia driver, updated to latest Nvidia driver.
2. Extracted the offline Opera One installer, rebooted into Safe Mode, and installed from within Safe Mode using installer.exe within the extracted folder, but used All Users during the install process this time, instead of just my personal user account.

And success. The browser installed properly. This was after clearing all browser related folders, using CCleaner to remove the broken uninstaller link in Add Remove Programs, and then cleaning the registry of any Opera related entries (CCleaner does this as well):

C:\Users%USERNAME%\AppData\Roaming\Opera Software
C:\Users%USERNAME%\AppData\Local\Opera Software
C:\Users%USERNAME%\AppData\Local\Temp* (specifically any folders starting with 7z or Opera)

I think it was either installing from Safe Mode, installing to All Users, or updating my graphics driver from 577.00 to the latest driver from Nvidia. I also use two apps, FanControl and SignalRGB, and both of these hook into various drivers at relatively low levels, so it's possible those were interfering somehow. Installing in Safe Mode likely bypassed any possible interference.

One, or a combination of those likely ultimately led to it working, but as I said before, I also had some issues with my file system being a bit corrupted (everything else on my system worked fine, so it must have been a very limited amount of corruption).

To recap:

1. Initial Checks

Verified that your user account (jivef) had Administrator privileges.

Confirmed Windows Memory Integrity and LSA Protection were enabled, other antivirus scans disabled.

Checked the TEMP folder permissions and owner (your account).

2. .NET Framework and Redists

Inspected Visual C++ Redistributables (2010, 2012, 2013, 2015–2022) and repaired the latest versions.

Checked .NET Desktop Runtimes (6.0, 8.0, 9.0) and Windows Frameworks (3.5, 4.8).

Renamed .browser files in Framework folders (opera.browser → .bak) to allow regeneration.

Attempted ngen.exe update /force to rebuild assemblies; some errors were shown for WPF speller, but proceeded.

3. System Integrity and Repair

Ran:

sfc /scannow – found some issues, but not fully fixed.

DISM /Online /Cleanup-Image /CheckHealth

DISM /Online /Cleanup-Image /ScanHealth

DISM /Online /Cleanup-Image /RestoreHealth /Source:X:\Sources\sxs /LimitAccess (used Windows ISO as source due to component store corruption).

Enabled additional .NET features required for Opera installer to function, e.g., ASP.NET 4.8 and WCF HTTP Activation (after DISM fix).

4. GPU and Driver Troubleshooting

Suspected Nvidia driver issues (recent update via GeForce Experience).

Used DDU (Display Driver Uninstaller) in Safe Mode to completely remove GPU drivers.

Reinstalled known stable Nvidia driver (577.00 initially; later considered newer stable versions).

5. File System Checks

Ran chkdsk /f /r on C: to ensure the filesystem was healthy.

Verified SSD health via CrystalDiskInfo (reported 90% health).

6. Opera-specific Steps

Deleted/moved all Opera-related folders:

%LOCALAPPDATA%\Opera Software\

%APPDATA%\Opera Software\

Any leftover Temp installer folders (7z*) in %TEMP%.

Removed .bak from opera.browser files after repairs.

Downloaded offline full installer and tested running it from extracted folder.

Installed All Users option when prompted.

7. Event Viewer Observations

Found repeated c0000005 access violation errors in ntdll.dll during installer runs.

LiveKernelEvent 141 suggested GPU-driver or memory-related hook issues.

Observed BEX64 faults related to the installer module.

8. Result

After cleaning out all remnants, using a stable GPU driver, ensuring .NET features were fixed via DISM, and using the offline extracted installer, Opera finally installed successfully.

Settings and caches are stored in:

%APPDATA%\Opera Software
%LOCALAPPDATA%\Opera Software\

Have you guys seen any issues with Nvidia driver 577.00 in regards to Opera One?

I've run sfc /scannow, I've reinstalled all of my Microsoft redists, DISM /Online /Cleanup-Image /RestoreHealth, used safe mode and DDU to uninstall my GPU driver, then reinstalled fresh. My drive health looks fine, as CrystalDiskInfo says 90% good. Tried emptying the TEMP folder completely. Checking permissions on TEMP folder, etc. Even tried installing it with a temporary custom temp folder through elevated command prompt with no luck. I've tried the offline installer both from the link given when the online installer fails and downloaded directly from the Opera FTP website. No luck..

When I unpack the full installer exe with 7zip, the browser runs just fine. No issues there. It's the installer that won't work, and I just went through and installed 5 other browsers with no issues (Firefox, Floorp, Chrome, Vivaldi and Brave all install and run just fine). First, I was getting side-by-side errors saying to use sxstrace, but then after deleting everything related to Opera in multiple different folders, I started getting the corrupted .pak error.

The only thing I haven't tried is sxstrace, but I ran that last time this happened and it came back with no issues.

This is the only program I'm having trouble with. Other browsers install fine. Games run fine. No issues. Any and all errors in Event Viewer are to do with trying to install this browser and something blocking it and causing an access violation in ntdll.dll. Out of dozens of apps, this is the only one I have issues with. And the issues only started after I installed VS Studio and its corresponding SDKs, etc. I removed VS Studio, made sure my redists were repaired, but still having this issue.

Edit: I ran chkdsk, it found some errors and repaired them, ran sfc /scannow and restore health again after that, which came back with no issues, but still getting corrupted .pak error on install. Not sure what's going on. I think I've just about exhausted my level of expertise on this. One thing in the event viewer made it look like Opera One was running into some issues with my display driver as it was trying to install, which is why I reinstalled that.

That will tell you if the issue is with your user account or system-wide. You can delete that test account when you're done testing.

Then, check your file system.

Then, check the health of your drive with CrystalDiskInfo.

If that fails too, then I doubt your issue has anything to do with existing Opera file and or Opera reg keys laying around.

just that reddit thing looks like its for opera

The main examples are for regular Opera, yes. But, there are notes in there for Opera GX too. Some things there are implied. For example, if it says to delete the "Opera Software" folders, you wouldn't do that if you also have regular Opera installed. You'd instead just delete the "Opera GX Stable" folder in the "Opera Software" folders. As another example, if it says to delete all Opera uninstall keys in the registry, you would skip keys for regular Opera and delete just Opera GX's keys. As another example, you wouldn't delete "HKEY_CURRENT_USER\SOFTWARE\Opera Software" if you have regular Opera installed too. You'd only remove Opera GX-related stuff in it.

With that said, I'll have to make things a lot clearer on that Reddit page sometime.

Maybe https://www.revouninstaller.com/products/revo-uninstaller-free/ will be easier and allow you to remove all of Opera's traces so you can reinstall.

Note that you can uninstall regular Opera or get rid of all its traces manually without affecting its data and settings etc. as long as you keep the "Opera Stable" folder in "C:\Users\yourusername\AppData\Local\Opera Software" and "C:\Users\yourusername\AppData\Roaming\Opera Software". You can even back up (copy to a safe place) the "Opera Stable" folders just in case any uninstall utility you use wipes them out.

For the .pak file issue, did you try using the offline installer from https://ftp.opera.com/pub/opera_gx/120.0.5543.85/ as suggested?

If so and you're still getting a .pak file issue, there's something corrupting your download or interfering with Opera's installer. What anti-virus and security software do you have? Also, what version of Windows?