What does this traffic sign indicate?

You answered correctly. This sign indicates a diversion route (Umleitung) for autobahn traffic when needed.

In Germany, the letter "U" followed by a number on a blue background is used to mark diversion routes during roadworks or closures on motorways (Autobahns). These diversions help guide drivers along an alternative path to avoid the closure safely.

It is not a sign for an underground station or a federal road — those have different symbols and colors. Understanding these diversion signs is important to follow temporary traffic changes without confusion or violation.