On https://ecoepi.miraheze.org (private), subpage functionality is broken in the main namespace

Symptoms:

• Linking to subpages via /Subpagename and /Subpagename/ is not working
• No breadcrumbs shown on subpage

Only the main namespace seems affected. At least in namespaces "User" and "Template" it is working.

I already tried switching subpages off and on for the namespace using the new "Manage wiki's namespaces" special page, but without success.

Probably the same issue as T4036

Thanks!