Important
This issue occurred because the location of SRTM 3Sec (AutoDownload) has changed, so SNAP is no longer able to retrieve it during Terrain Correction, Back Geocoding, Topographic Phase Removal ect.
As a solution:
- switch to SRTM 1Sec HGT (AutoDownload) or
- change the location of the data as described here
This will be fixed with the next SNAP update, so please keep your version updated (Help > Search for Updates)