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Using the command-line rules out a graphics driver issue. “Updates not found” suggests a network problem. Are you using a proxy? If you are using IPv6 and can switch to IPv4 that is worth trying. Does “Reliability Monitor” show any issues at the time SNAP fails.
Try .nbm files for offline install . If you can’t download the updates with a browser try a command-line method (curl or wget) which give lots of debugging parameters to pinpoint the issue.