We are experiencing a critical issue in Copilot Studio where all existing agents created with the legacy Copilot Studio UI return 'Something unexpected happened. Error code: SystemError' across multiple environments, including both sandbox and production (non-default) environments. The error occurs immediately when opening the chat, with no prompt required, and happens in both the Copilot Studio Test panel and published channels such as Teams or Web. This affects agents with no knowledge sources, flows, or connectors, and re-publishing or saving the agent again does not resolve the issue. New agents created in the new Copilot Studio UI work correctly, and legacy agents still work in the Default environment, but legacy agents fail in all non-default environments. The agents were working until recently and stopped at roughly the same time yesterday 11.06.2026, indicating an environment-specific and UI-version-specific issue. The problem is not related to flows, connectors, or data sources, and impacts about 90% of existing agents, affecting both testing and productive usage. This suggests a runtime or backward compatibility issue affecting legacy Copilot Studio agents in non-default environments, possibly related to agent initialization, runtime orchestration, environment-specific execution layer, or legacy agent schema compatibility. We are seeking information on any known issues, runtime differences, recent platform updates, migration or re-binding processes, or if this is a regression or intended deprecation behavior.
Long version (sorry for the formatting mess)
