Ok, it appears the “Cause of failure could not be determined” is related to the caching I tried to introduce to speed up the build (usage of $XDG_CACHE_HOME environment variable - see script in linked post below).
I commented out the cache saving (rsync of built files to the $XDG_CACHE_HOME destination directory) and the build + deploy has succeeded - but so far only once, so who knows whether it will stay stable.
Maybe a very large cache directory causes these types of “unknown” failures?
So @tyler would still be good to see if you can take a look at it, even though it succeeded once.