How to fix System Error

I faced similar issue before to my Copilot Agent (but only at the published agent, not test mode), and found out the reason is that we do not have the proper start for the conversation. We created a Custom topic for Greetings and set a few starting words, then direct it to 'Start of Conversation' (original trigger of this should be On Conversation Start) then the agent works well. It's not necessarily the same reason of what you are facing, but you can try it out.