Townhall works fine - generated cities are OK. But the city_road problem persists.
I found out that the same problem is with non-navigable rivers. But when I changed one of them to be 1 km/h and 1t, then it became buildable. But the street is 50 km/h 40 t
Found and fixed. There are placeholder tracks for monorail, maglev, tram, air and narrowgauge which all had the same name=Se_road_50. I have renamed them and the road now works as expected.
River problem was probably that a river with 0 cost, 0 speed, 0 maintenance, and 0 whatever, is considered a mothballed river, and thus cannot be built. Only as a downgrade of exisiting river. Fixed by setting construction costs for all rivers to be cca 10x higer than roads.