XF 2.3 Admin CP UI issues: Development menus and Admin permissions "flashing" then disappearing

[FrancescoL]

Hi everyone,

I am experiencing a critical UI issue in my XenForo 2.x Admin CP that is preventing me from managing my custom add-on development.

The Problem:Even though I am a Super Administrator and have enabled development mode in src/config.php ($config['development']['enabled'] = true;, several UI elements "flash" for a fraction of a second and then immediately disappear.

Specifically:

1. Development Menus: When I click on the "Development" section or try to access the "Import" action for my add-on, the options appear briefly and then vanish, leaving the page incomplete or showing a "Requested page could not be found" error if I try to access the URL directly.
2. Admin Permissions: When I try to edit my own administrator permissions to ensure "Manage development items" is checked, the overlay/window with the checkboxes appears for a millisecond and closes itself instantly.

What I've verified so far:

• $config['superAdmins'] is set correctly to my User ID.
• The _output folder and addon-output.json exist and have correct server permissions (755/644).
• I've tried different browsers, cleared cookies, and used Incognito mode, but the "flashing" behavior persists.
• I've checked the browser console (F12) and I see [mention here if you see any red errors, otherwise say: "no obvious JS errors"].

It feels like a JavaScript conflict or a server-side permission check that is failing immediately after the page loads, causing the UI to hide these elements. Because of this, I cannot import my routes or templates from the filesystem.

Has anyone seen this "disappearing UI" behavior before? Could it be related to specific PHP settings or a core JS issue?

Any advice on how to force the Admin CP to stay "unlocked" would be greatly appreciated.

Thanks in advance!

 

[StarArmy]

What browser are you using? I have a member that says a similar behavior is happening to her when she clicks links to my forum from Discord. The forum will appear for a moment, then "flicker" and vanish leaving only the background color of the page. She says she is using the Duckduckgo browser. I haven't been able to replicate it but I have a video she made of it.