I've opened sump housing to check for clogging but it was clean. When I removed check valve to inspect it I've found out that the round part of the valve was missing so I figured it might've blocked the water exit at drain hose or something like that. So I pulled the dishwasher out to get a full access to drain hose connector and the pump. Upon disconnecting the drain and checking its connector I indeed found the round piece totally blocking the hose and acting as a seal. I removed the piece with pliers and installed replacement check valve. Since I've ordered a newer and better pump I also decided to replace it. Now my dishwasher works like a brand new.