Content like

<q cite="https://www.gesetze-im-internet.de/urhg/__2.html">

results in an error message in ebpubcheck 5.0 like:

Remote resource reference is not allowed in this context; resource "https://www.gesetze-im-internet.de/urhg/__2.html" must be located in the EPUB container.

I could not find such a restriction in EPUB 3.3.
With href of a this generates no error message.
Of course, one can point to a bibliography entry with the cite attribute, but is this really required, intended or a bug?