Now I downloaded QuadPol data from ESA and i also received the error at the Terrain Correction step:
I noticed that the footprints of ESA data are not as long as the products from ASF. The ESA data are, as stated in the error message, already detected. I find this quite weird because they were downloaded as SLC products. Obviously, there are two kinds of SLC data in the ESA archive:
Solution 1:
(credits to @gusortiz who originally found this workaround).
If you don’t have much topography you can just use Radar > Geometric > SAR Mosaic with only one product. Uncheck “weighted overlap” and “normalize” and run.
This has basically the same effect as Ellipsoid Correction because no terrain is used for orthorectification. In my example layover and foreshortening are clearly visible and obviously not corrected. But at least geolocation of the whole product is correct.
So this only works for flat terrain!
I will test if the different SLC products make any difference in SNAP.