Update necropsy results: got the results today and spoke to the leading vet of the pathology institute: No Avian Flu. No Newcastle disease. They also checked for "common bacterial diseases" he told me - nothing.
The hen they examined happened to be egg bound (that was her first egg after hatching chicks and it seems it got stuck - but she never showed classic signs of it, just died suddenly).
And that doesn't explain why 5 of my chickens died within two weeks and the 6th one is about to die. When I reminded them of that, they were a bit worried and said they didn't have an explanation and offered to do a necropsy on my next chicken that dies for free.
The next candidate is my second roo, the one with the blueish shriveled comb in the pictures I posted earlier this week. Poor boy, he's been getting worse and is now so weak that he almost can't stand or walk any more. Actually I'm not sure if he's partially paralyzed or just extremely weak. Tried to give him some soaked bread and water right to his beak, he took only one bite.
It's evening now here and I think he won't make it through the night. Very touching thing - he has somehow managed to limp up to his one hen who is broody on eggs and he's now snuggled up against her (they have been living so close that she's anyway been exposed to whatever it is). He's never slept in the broody corner before - obviously doesn't want to be alone in his last night.