Despite what some people will tell you, this is a common issue with these VW picnic chairs. There is a design fault in the plastic joint shown in your photograph. It fails because this cheap plastic component has a hole through it to take the securing pin/rivet. The hole provides the built in weakness. And before anyone starts banging on about the weight of the person using the chair just bear in mind that this part isn't load bearing. It merely joins the arm to the top of the front leg frame.
You can buy metal replacements for these joints but before going down that road do you have a Factory Extended Warranty? If you have one then VW will replace the whole chair. The FEW offers exactly the same benefits as the original three year warranty despite what the service manager might try telling you. Don't take "it's not covered sir" because it is. VW Customer Services have told me as much in writing.
I've had two go and I also think that they aren't fit for purpose in their factory form. I also don't find them at all comfortable. Accordingly, I don't use them.
The replacement metal joints were being made by DomiT6, a forum member You can contact him via his Email
D.Baeurle@gmx.de Use the search function and type in "Folding chair broken" then take a look at post 34. He has posted some great photos.
He is a lovely chap and very helpful.