From clean install to update when opening SNAP: probably too many things to be updated/too much time on server?/server too busy?
THE Only solution was here: Install a Toolbox via Plugin Manager - SNAP - Confluence (atlassian.net)
THE only solution was to skip any questions and install from within SNAP via Tools, Plugins, SNAP, toolboxes all things box by box, bit by bit. Also there several retries necessary…
I am pretty sure your server is way too busy.