Custom Agent Builder agent suddenly failing with "Oops, something happened" - no config changes

Hi All,

We built a custom agent in Agent Builder (Microsoft 365 Copilot) that generates a Word risk assessment by grounding on a contract (either the document currently open via the Word Copilot add-in, or a file uploaded in the web browser chat), plus a fixed set of knowledge sources (guidance notes, playbooks). It worked reliably for about 5 days while I was testing it solo. We then rolled it out to appr. 20 users, and shortly after, it started failing consistently with a generic "Oops, something happened, try again" error, typically after a long delay, in both the Word add-in and the browser version.

I can't confirm causation, but the timing lines up closely with when we expanded from single-user testing to ~20 concurrent users. I don't know whether this points to a tenant-level capacity/concurrency limit being hit, or its something else, just stating it in case others have seen a similar threshold effect.

Is this a known concurrency/capacity limit for Agent Builder agents once usage scales up, or a separate backend regression affecting document grounding, and has anyone else seen this pattern after moving from single-user testing to broader rollout?