Matej
Looks to me like a classic case of uneven print development as Aggie has said. I would look at you technique when first immersing the print into the tray. It is essential to get the whole surface submerged without air pockets. Somebody suggested the rocking of the tray first and immersing the print in the wave of dev. Give it a try and make sure you have enough developer to fill the tray to at least 1 cm deep as well
Good luck

Phill