Thank you for providing details, @jj1! Based on the error it seems like when rejoining the webinar, the user is not being identified. Can you share if the user rejoins the webinar via the same link or is the tk parameter used ? If not, can you try rejoining the webinar with the registration token and see if the problem persists?
1.4 Meeting error codes
https://marketplace.zoom.us/docs/sdk/native-sdks/iOS/resource/error-codes/