It is the same error mentioned in this post. Unfortunately the post that the linked reply points to is gone now so I don't know what the solution that worked "...around some limitation with Nautilus" actually was. If someone here happens to know then maybe that might solve the problem for me.
I'm fairly sure it has to do with setting authentication and there's usually a Samba panel somewhere that this is configured but I can't find it, even as Gnome extension.