Just an observation: to me it looks as if adc2 is still on 2.6.1+samba4.17.3+dfsg-1~bpo11+1 for samba packages I'd have to check for all related packages step by step. I DON'T want to upgrade adc2 now, as long as it is the only working DC. - adc1 is on 2.6.1+samba4.17.3+dfsg-2 already. Maybe that is part of my problems.