Rudi, I believe it is used as UA RCC for UA's own morning departure and then LH uses it as SEN/F lounge for at least the evening departures.
Back in March when I was only an FTL, I remember it being closed with no sign on my way to the Business lounge. When I got out, it was opened with the "Senator Lounge" sign. This past august when I was connecting to LH745, it was also open with this LH sign at around 11 pm.
Maybe I'll try my luck at the LH Business lounge on Tuesday.