Acoustics can be tricky... If your technical approach was identical, there must have been less input level from the singers and musicians, maybe less reflections in the room/more dampening, maybe more or less humidity, maybe more distance of the singers to the microphones... 15 to 20 dB is quite a heavy difference, but 10 to 15 dB is absolutely normal. You should have noticed it at the location already during sound reinforcement. But you should compare the signals in the recording program, not Studio One. Capture can not alter the levels, so if there is a difference, it was caused by different acoustics in the rooms.

Hi wahlerstudios
Thanks for your reply. It was definitely not the acoustics, and I'm sure I would have noticed 15-20 dB difference during the live performance..
After some hours of extreme frustration I switched - just for fun !!! - the driver from Audiobox to Firestudio and back. Suddenly all the levels came up to what I expected. I have no explanation why that happened only with one recording. I would expect a driver issue to happen on all recordings or none. Luckily it's ok now. But I would be more relaxed, knowing the reason for this very strange behaviour, which I never had before. Thanks anyway.