Views - Contextual filter -  "page not found"
This is an unfortunate side effect of how the menu system works. When the paramter "%" isn't available, you match against a different path, and the View won't get called.

I suggest making /objects an alias to /objects/all, I believe this will then interact properly with the menu-system.
If that's not the case, you can force the issue with Path redirect which will redirect the user with an HTTP-response code, and thus land the user at /objects/all.

I minor drawback of using Path redirect is that the redirect logically belongs together with the View, but is now configured somewhere completely different. For that reason, I would prefer to accomplish the redirect and View rendering with Page manager, which will allow you to group the HTTP-response and a Panel together in a more logical manner.