Ok, since the workspace fell into an endless ‘archiving’ loop, I managed to salvage the sources (before it started ‘archiving’ again) and just created a new one. For anyone in the future struggling with this, here are a few tips:
Make sure to backup sources to your local machine before messing around with system packages(!)
apt-get upgrade. The warning says that it is not recommended due to space constraints, but I’d also add that it is outright dangerous, since my workspace failed once I ran the upgrade and decided to add more space.
If you do need to upgrade/rebuild some packages (which in my case was adding support for OpenSSL instead of GnuTLS for Git, which was unfortunately required by AWS CLI credential helper - see here, which by itself would have already been satisfied if the workspace ran on 16.04 instead of 14.04), then do that piece by piece, and don’t run (1) first.
Hope that helps.