They were in a holding pattern over Lübeck, which is quite close to Hamburg. I haven't seen mention of the weather at Lübeck at the time - was an approach and landing there feasible?
If so, why did the controller not suggest a landing at Lübeck?, It's not clear from the report that the crew were aware that they were dealing with a front rather than isolated convection, but the controller should have been, and should have known that both Bremen and Hannover were behind the front.