Hi, I have
- unistalled snap & all user data using the snap-8 uninstall script,
- then realized, that updates are stored in the directory of the user doing the update,
- manually removed ${HOME}/.snap
- reinstalled snap-8 as root.
The result is: I can open Sentinel-3 SY-2-SYN products in SAFE format by clicking on the manifest file again. I cannot speak for the original poster, but my issue is solved (and also the re-post here https://forum.step.esa.int/t/error-opening-s3-syn-safe-format-with-snap-v8/27456. Thank you!