That is *all* the crazy.
Hydrogen is very very deprecated and non-active... To be honest I'd be a bit scared to dive back into that codebase. I'd recommend downgrading PHP on that machine if you have no other choice but to keep running it. If that's not an option... hmm. Just based on that string above it might be worth it to look at your encoding settings? I haven't touched PHP for anything more than light maintenance in quite a few years, so my rust is thick.
In the end, I think the issue was a conflict of namespaces. I was running another system from inside Hydrogen. When I moved that other system's namespaces to the top of my index.php file it started working. But in the end I just didn't have enough time to commit to the software upgrade I was doing and had to roll back to a backup.
I built my current site on Hydrogen and now I'm working full time (in another industry) I don't have time to change. But I love Hydorgen. I'm building a really cool system at the moment (maybe my last), which maybe I'll post here when I launch it live