Black deposit on the solder after the vinegar cleaning run?

[PuffingBilly]

I've made the copper pot "moonshine" still but when I made the first cleaning run with the vinegar (I used the diluted glacial acetic acid) I've discovered that the internal surface of the pot is covered with the black streaks and the solder has the black "oily" deposit, which are hard to remove (I tried citric acid with a little success).
To make a stil I used the tin-copper solder (lead free) - Sn97Cu3, which is used for plumbing usually. And I didn't expect to find the deposit, which looks like a car grease (see the picture).
What did I wrong? Maybe I used the wrong solder?