This error is generic and it is raised when HTTP/2 protocol is enabled. If you do not use HTTP/2 you can disable it in the browser settings of your populations. It won't fix your issue but you will probably get a more meaningful error message.

The question is do you get those errors with lower load? Also have you checked if those errors are raised by all your LG's? If it's the case then it indicates that the issue is between your LG's and your application. The only way to narrow down the issue is to use an LG in a different network path near your application. Then check if you still have issue from that LG. Depending on the result you will be able eliminate some of the network devices like firewall, Load Balancere etc...