A user, haydenjames, encountered an error when returning to the forums after clicking admin settings. The issue was also reported by merefield and Arkshine on recently upgraded production installs.
After investigation, Arkshine found the error to be introduced by a GitHub pull request (#24373) and occurring in the discourse/app/routes/discovery-index.js file. The issue happens when the urlMethod is replaceURL and the url field is missing.
A fix has been implemented by david, and users are advised to run an update via /admin/upgrade or the console to resolve the issue.
To test I have uninstalled all the themes (1 additional) and components (3) and removed Cloudflare (all .js is already bypassed using cache rules but just to be sure I removed it temporarily) then tested in Incognito mode. But the same result
Update:
I tested with safe mode enabled. (all 3 boxed checked) and still the same error:
Thanks everyone - this should be fixed in tests-passed now. @haydenjames if you run an update (either via /admin/upgrade, or via the console), things should work much better.