I recently started a PhD in Germany.
To broaden my knowledge in my field (Computational Sciences), I thought it might be a good idea to visit from time to time the institute from which I received my master's degree (another university also in Germany). There are many PhD students who I know and it would actually help me a lot for my own studies.
However, after giving me a warm welcome the secretary of that institute said that I am not allowed to work there anymore as I am not an employee of that institute.
On the other hand, I asked both professors, and none of them seems to have a problem with it.
So, while I admit that my approach is a bit naive and on industrial level I would of course not really be surprised with such a reaction, is there a certain procedure that I need to follow? Do I need to hand in an official request? Or are PhD students of different institutes working together on one's own not well regarded in general?