In short, both stories are somewhat true. Stade Roland Garros was an internment camp for political dissidents and Jews (most prominent among them, Arthur Koestler). But it was an internment camp only from September 1939 to June 1940.

From 1941 to 1945, the French Tennis Federation (FFT) did not hold a French Open but did hold an annual Tournoi de France at Roland Garros, which was strictly limited to professional tennis players of French nationality. Today, the FFT does not recognize these tournaments as part of the French Open's history.

I don't have any sources that discuss the Tournoi de France's popularity among the French, but I think it's safe to assume that it was a fairly popular event. Historians Julian Jackson and Robert Paxton have each described how sporting events were key aspects of Vichy cultural life--they both instilled nationalist pride and took the population's mind off of the embarrassment of Nazi occupation. Tennis' significance in Vichy France is also indicated by the fact that Jean Borotra, one of France's most famous tennis players, initially held the title of Vichy's Commissioner for Education and Sports.

Sources:

I'm an avid tennis player, and try to read tennis history when I have the time. Unfortunately, there are very, very few histories of tennis done by trained historians. That being the case, the best source I can find on this topic is this Atlantic article. If you'd like to understand the significance of sports in Vichy France, I remember Julian Jackson spends a good bit of time discussing it in his book, France: The Dark Years, 1940-1944