Steps To Reproduce

To reproduce the behavior:

1. Create a node with an alias. Let's say its /my/path/alias
2. Create a menu item for the node in the Structure->Menu UI
3. Set the menu item's path to my/path/alias and see that works fine
4. But now set the menu item's path to include an anchor: my/path/alias#anchor
5. You'll receive an error that my/path/alias doesn't exist or you're not allowed to access it
6. Try all the steps above, but now use node/[nid] instead of the alias. It works with an anchor.

So the problem seems to be that whatever validation is happening when you add a path alias as a menu item is failing if you include an anchor.