DropBox link error

I used this app from the beginning. I love it. I am now on my 3rd model of iPad, with the Air 2.
I installed the app, I already had DropBox installed and connected.
I tried [multiple times] to connect PDF Pro to DropBox with my account email and password, it starts or tries to link [after a few turns of the pin wheel] and then it just freezes. I can’t do another thing. I actually have to force stop the app to get back into it again.

DugRut about PDF Pro - Reader Editor Forms, v2.4.1