I've seen this occur when the GeoTiff is in a spherical projection (e.g. Geographic Lat/Lon/WGS84 or similar). Currently TerraForm wants the GeoTiff data in a planar projection, such as UTM. This is because TerraForm uses the projection of the source data to set the geographical location/projection of the landscape actor (which must be planar in UE4).
If you double click on the DTM layer in QGIS and pull up the 'Information' tab, it should tell you the Coordinate Reference System (CRS). Please could you advise what this is currently set to?