Code producing error, only on third run of Userform...?

I have created a Userform with various Listboxes, some interacting with others.

I'm currently getting a rather frustrating error that decides to only show up on the third run of the UserForm.

It will work perfectly fine with two runs of the Form (as in open, close then reopen) however, on the third attempt it will throw up an error (as attached) and will continue to do this for three runs of the Form and will work on the fourth.

I'm not changing any piece of code within these runs, I don't understand why it works one minute and not the next...

As mentioned, I have attached an image of the issue I'm having. Hopefully this provides enough info...

Thanks for your time.