Hmm, according to the devtools output, gnuplot is an uninstalled dependency. I just can't find the reference to it anywhere, so it might be a red herring. It's removed for the time being.
I'll push an update for 1.6.3 in a few minutes and include the move of the {SESSIONKEY,APIKEY}.txt to /etc/etherpad-lite.
Please keep that change in mind!
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