Pigeons???

Well, I just can tell you the meaning of the pigeon that $hits on Karol.

According to Kieslowski that is a way to introduce Karol's character as a pathetic fellow that even before being humiliated by his wife is humiliated by a simple pigeon.
That pigeon $hitting on Karol before he enters the court room is telling us what is going to happen to him later in the movie.

About the pigeons in the rest of the movie, well I am not sure about its meaning, perhaps they are there to remind us what happened to Karol earlier.

I might be wrong, though.

i also noticed the recurrent symbol of pigeons and the sound of their flapping wings. Can it be symbolizing 'liberty' - the theme of of the previous movie?

I don't have a solid answer to what the birds may symbolize. For me they drew attention to certain details which I came to appreciate by the end. The pigeons in the wedding scene at the beginning lends a kind of simple beauty to the scene. But they also proved to be a point of ugliness or disappointment, as when one of them defecates on Karol on his way to the divorce court. I think the pigeons represent a thing of beauty and of ugliness, and they serve to draw a parallel of the beauty and utter disappointment that Karol constantly finds when trying to deal with Dominique. Also, just as Karol can not let go of his feelings for his ex-wife, the pigeons are at many locations Karol is at. Even when you don't outright see the pigeons up close, you can hear one flapping its wings in the background or see them flying high above. It's suggesting that Karol can not rid his mind of Dominique, even though she jerked him around often. The motif works because both the pigeons and Dominique are established early in the film to represent both beauty and disappointment to Karol.