Actually the skywatch bucket is only used as a fallback.
If the data is not found at the URL provided in the snap.auxdata.properties, then https://download.esa.int/step/auxdata/dem/SRTM90/tiff/ is used. Often the same. If this doesn’t work either then the skywatch bucket is used.
So the first problem is that the download from https://download.esa.int/step/auxdata/dem/SRTM90/tiff/ is not working.
But access vie this link is working: https://download.esa.int/step/auxdata/dem/SRTM90/tiff/srtm_36_02.zip
Have you maybe download a lot of data and requested a lot of DEM data in parallel?
Maybe you have been black-listed.
See also this thread: Snap connection to server
@Oana, can you check this?