sorry, my fault. I "had to" upgrade ffmpeg at some time in the past;
i ffmpeg Depends libavformat58 (= 7:4.3-2) i A libavformat58 Depends libgnutls30 (>= 3.6.14) i libgnutls30 Depends libc6 (>= 2.25) and so I had a newer libc than stable. Sorry about the confusion; perhaps a "Conflicts" might be a good idea, though. Thanks for your effort!