When switching the interface language on any topic while logged in as admin, a permanent error message appears: “Failed to preview theme because it doesn’t exist, or you don’t have permission to access it.” Regular users do not experience this issue.
Steps to reproduce:
Log in as admin
Open any topic
Switch the locale using the language selector
Error message appears permanently — not briefly
With regular user account — no error.
Discourse version: 2026.1.3
Do you get this issue on a different browser with the same admin user?
No. Only Chrome. And this problem only appeared today. I just checked and other browsers don’t have this problem.
Do you see a ?preview_theme_id=ID in the browser’s address bar at any point?
This error message is only shown if you’re previewing a theme with an incorrect ID. You likely did this at some point on this browser and that value “stuck” in the Chrome tab/window.
Issue resolved! It was caused by a stale ?preview_theme_id=6 parameter in the Chrome browser URL. Once removed, the error disappeared. Thank you for pointing that out!
