bizarre issue where error for cache reload going over menus if one links to internal file pdf

The menu link works fine.

The file is definitely there, and can be visited by following the menu link.

Something in the router system fails to find it and throws the resource not found exception, which can interrupt a module saying it successfully installed (even though it does successfully install) and maybe other issues, but is not noticeable in regular operation of the site.

The path that it claims is not found is the file path, and again, the file is there, but Drupal should not be expecting a file path to work like an internal route.

(It announces this in a confusing manner, naming the path that is being visited when the error occurs not the menu link path, but Drupal does have an issue to fix that: Wrong path in exception ResourceNotFoundException [#3327845] | Drupal.org)

Closest thing to a related core issue we could find was this: Combination of language negotiation and path aliasing can cause a corrupted route cache, 404s [#2802403] | Drupal.org

and that’s not it.

Vaguely in the ballpark, but not it: A non-existent route breaks the site [#2938884] | Drupal.org

Maybe related? Error when the menu contains a link to a taxonomy term page [#3251675] | Drupal.org

We did check, our front page is set to a sensible path, it’s not whatever this is about: Multilingual Site, Homepage 404 Error after migration: None of the routers in the chain matched url ‘/’ [#2804957] | Drupal.org

Maybe maybe related? Links widget reset link error on translated nodes [#3108498] | Drupal.org