Last time I played Dwarf Fortress, I tried to reroute a river through a dolomite block canal, but two tiles produced a "to near to edge" warning while trying to build the walls.
That shouldn't happen since tiles are underground and next to an edge wall, not free edge.
After testing for some time, I figured out that it should be a new bug. As far as I know, building walls within four tiles near an free edge (above ground) is forbidden, though it works undergound next to an edge wall.
Bug: If you channel natural floor four tiles next to free edge or if floor is missing, (due to river, cut tree over hall, etc.) the tile one z-level below is marked as next to free edge and walls cannot be build on it either. This can continue for multiple z-levels if you continue to channel natural floor. Due to some reason ice floors are regarded as missing natural floor and can create the same problem even if not channeled.
I guess creating new "natural floor" would work, but I don't even know whether "natural" floors can be built.
Any idea how to solve this problem?