after updating Opera to 136.0.6008.52, Opera stopped starting with my existing profile on Pop!_OS 24.04 LTS.

Starting Opera from a terminal resulted in:

Illegal instruction (core dumped)

and no Opera window appeared.

System Information
Opera: 136.0.6008.52
OS: Pop!_OS 24.04 LTS
Kernel: 7.1.5-76070105-generic
CPU: Intel Core i7-14650HX

What I tested
A completely clean profile works normally:

opera --user-data-dir=/tmp/opera-test

I then found that renaming:

~/.config/opera/Default/Preferences

also allowed Opera to start normally. Opera generated a new Preferences file by itself.

This showed that the problem was somewhere in the old Preferences rather than in Opera itself or the entire profile directory.

I compared the old and newly generated Preferences files and tested individual top-level JSON sections by transferring them between a working and non-working configuration.

After narrowing the problem down, I was able to reproduce the startup crash with the old rich_wallpapers section.

In particular, my old Preferences contained:

"rich_wallpapers": {
    "revert_wallpaper_time": "0",
    "selected_id": "mods/mcidbianigdmjfmianaeckfdfpgihncn/default",
    "takeover_wallpaper_id": ""
}

The newly generated working Preferences instead used:

"rich_wallpapers": {
    "revert_wallpaper_time": "0",
    "selected_id": "bundled/default"
}

Replacing the old rich-wallpaper selection with:

"selected_id": "bundled/default"

allows my original Preferences file to work again, while keeping the rest of my previous configuration, including my sidebar and UI settings.

Restoring the old rich_wallpapers setting reproduces the startup crash.

Steps to reproduce in my profile

1. Use the old Preferences file with:

rich_wallpapers.selected_id = mods/mcidbianigdmjfmianaeckfdfpgihncn/default

2. Start Opera 136.0.6008.52.
3. Opera terminates during startup with:

Illegal instruction (core dumped)

and no browser window opens.

Expected behavior
If an old Mod wallpaper is no longer valid or compatible, Opera should ignore it or fall back to the default wallpaper.
Actual behavior
Opera crashes during startup and the browser cannot be opened until the rich_wallpapers preference is changed.
Workaround
With Opera completely closed, changing:
rich_wallpapers.selected_id
from:
mods/mcidbianigdmjfmianaeckfdfpgihncn/default
to:
bundled/default
in:
~/.config/opera/Default/Preferences
makes Opera start again.

I have also submitted this crash through Opera's crash-report form.

Additional observation: after recovering the profile, Opera starts normally, but several pre-installed Mod wallpapers no longer work. The terminal shows repeated content_verify_job.cc:489 errors for Mod resources, including wallpaper/dark.jpg, wallpaper/light.jpg, music/music.ogg and multiple files under sounds/. This suggests that the startup crash may be related to an old selected rich wallpaper whose Mod contents no longer pass Opera's extension content verification after the update.