I've just replaced my tuners with light-weight Hipshot tuners to get a better balance, so the cause might be the tuners, although this seems rather unlikely. I stringed the bass without any problems, except for the G-string: for some reason the silk wrapping kept was thorn from the string and the metal core slipped out of the string hole. I've repeated this several times, each time cutting the string a little shorter than the remaining silk wrapping. After five or six attempts the string was simply too short to be used.
I'll report the problem to Thomann and hope they will send me a new G-string with silk wrapping that remains wrapped around the metal core. I gave the sound three stars, as I don't have any idea how the strings sound, but that particular G-string was obviously defective.