AppSync endpoint ERR_CONNECTION_CLOSED issue

The ERR_CONNECTION_CLOSED error usually suggests that a network connection was closed for some reason. This could occur due to several reasons such as network instability, the client or server closing the connection unexpectedly, issues with your AppSync configuration, or even client-side code problems. Here are a few steps to troubleshoot and possibly resolve this issue:

• Check Network Stability: If the issue is happening to a few users and only occasionally, it could be due to unstable network connections. Try to find out if there's a commonality among the affected users, such as geographical location, ISP, or connection type.
• Investigate Client-side Issues: Check your React application's code for potential issues. Are you correctly managing AppSync client instances? Are you accidentally closing connections or not handling reconnection properly when users switch widgets?
• Review Server-side Configuration: Look at your AWS AppSync configuration to ensure it's set up correctly. If there's a problem with your AppSync setup, such as an incorrect VPC configuration, it might cause connection issues.
• AppSync Service Limits: Check if your requests are exceeding any of the AppSync service limits. If you have a lot of users and you're hitting service limits, that might be causing your connection issues.
• Error Logging: Incorporate error logging in your application, if not done already. This could provide more insights into when and why these errors are occurring.
• Web Browser Issues: The problem could be related to specific web browsers or browser versions. If possible, try to find out what browsers the affected users are using and see if there's a correlation.
• Examine AWS CloudWatch Logs: If you have set up AWS CloudWatch for your AppSync API, you can check the logs for any potential errors or warnings when these disconnects occur.

Remember that this is quite a broad issue and might require thorough investigation. Ensure to do systematic checks at every level (client-side, network, server-side) to identify and rectify the issue.