
Good news: this isn't something you broke, and there's nothing to fix on your side. Your own findings point straight at a Microsoft-side issue. Every agent in the environment fails, including brand-new ones, and only this environment is affected while your others work fine. That can't be a config problem, it's the platform.
The Early Release cycle is the key detail. Those environments get platform updates ahead of the standard ones, so when Microsoft ships a bad change, that ring breaks first. That's why only this environment is down while your standard-cycle ones are fine.
And you're not alone. Others are reporting the same SystemError starting June 11, and Microsoft Support has acknowledged it's affecting multiple customers, so it's a known service-side incident.
What I'd do: