Unable to move a bug to a project without a version associated

I don't think we're ever going to create projects with default version numbers, this could lead to some confusion. I assume that you are doing this to move the "Closed" bugs to another project as you've mentioned before. But the filters have gotten much better, do they not help you achieve hiding "Closed" bugs?

As I told you in another discussion the reason is that "filters" are not "persistent".
Reasonly, Closed bugs will not be open or read anymore, but now still remain in bugs list. Everytime that I open project, I see all tickets, including closed one, and I should filter that. Then I open detail of a ticket, come back to list and again... closed tickets are visible, so I should filters again...... If filters would be persistent across sessions, could be enough but is not so.

In addition, this need is risen today not because I move closed tickets to a specific project, but because at the moment is not possible to open sub-tasks. I have many tickets related to one topic or funcitonality, so I created some projects that I consider "sub-projects" to move relative tickets, in order to better classify them.