Michal, hello! I am not the right fit for the customer interview, but I checked out your product out of curiosity.
I am not sure at what stage you are with your app (are you still ironing out the bugs?), but wanted to let you know that when I tried adding comments on cards, every comment appeared 4 times. There were also glitches (button text overflowing, columns not being added) when adding too many columns to the board. I am on Chrome/OS X Yosemite
The refresh icon to the top right was spinning all the time, which was puzzling (I wondered - is it synching something and never manages to?). I was also slightly annoyed that there was no way to close the black feedback box completely, only to collapse it.
I loved how the board tiles reflect the layout of the cards inside! Very neat design feature. Also, good job on the landing page (I would add a bit more copy explaining the features) & the domain name - simple and straightforward.
Wish you all the luck with the project! It looks great.