Uups, sorry. Problem solved.
Missing "NodeBB is rebuilding front-end assets" was the problem.
Wow, v0.6.0! That's going to be a fun one.
What I'd recommend is deploying a copy of your forum to a new environment purely to run the upgrade on it, then you can handle any errors that come up, and we can guide you through the process.
Now, of course, ideally, there won't be any errors, but v0.6.0 was a long time ago....