Hello, does anyone know the reason for the black line in the overlaps in the same way?
In all interferences this way؟ 
did you apply topographic phase removal during the interferogram formation and based on which elevation data?
SRTM 1Sen HGT
can you please try to download a DEM and use it as an external DEM?
These triangles were reported in various cases, yet their source is unclear.
Thank you very much