Recently I encountered SRTM download problems from http://step.esa.int/auxdata/dem/SRTMGL1:
SEVERE: org.esa.snap.core.dataop.dem.ElevationFile: Server returned HTTP response code: 403 for URL: http://step.esa.int/auxdata/dem/SRTMGL1/…
It seems to be an user access problem (error 403) to http://step.esa.int/auxdata/dem/SRTMGL1, and also other folders in http://step.esa.int/auxdata (I got errors for downloading orbit data as well).
Downloading 3" SRTM tiles works without problems, as these tiles are downloaded from a different source (srtm.csi.cgiar.org)
It is not clear why ESA changed access to their SRTM 1" HGT and orbit directories. A work-around is downloading 1" SRTM tiles yourself and put them somewhere, nevertheless autodownload will be preferred.
The version of SNAP, I am using 6.0.1 and tested also older versions, did not make any difference, the ESA server is the bottleneck I suspect.