After reading How to import an external DEM into SNAP? - #19 by Parisa I thought the issue might have been that the DEM I am using is in projected coordinates UTM33N but saving as WGS84 and processing again the whole sequence leads to the same elongated shape after Terrain-Correction. Could it be related to the high latitude (which is the reason why I cannot used SRTM)? although I cannot imagine a relationship between latitude and DEM deformation.