Vaadin : Error in case Performance load, 10% request fails get error page

I have vaadin application running with 8.1.5ver. When we run performance test with say 50+ concurrent users, 10%+ request receives error page(as configured in CustomizedSystemMessages extended class). This is deployed on liberty server.
Typically when load is higher,and first time running after restart, few request are never served after few clicks for those request. Rest of request(90%) are served properly with success. This percentage goes up/down a bit but this is consistently happening.
I am testing it through selenium so if error comes, i get to see redirected error page in screen shot.
There are no error at all in the server log files so there is no clue why it has failed.

Error is not coming at login page but at a certain step after user click.Further, This control never comes to developer written code( figured out from logs). It never reaches to the actionPerformed() to process the click for that failed request. It breaks before that and redirects to error page somewhere at vaadin framework level.
All i see is failed request with error page and no logs.
Everything works well when actions done manually.

Has anyone faced such issue…? Any clue would be very appreciated.

One guess here …there is a exception in vaadin framework level code (occurred before it invokes explicitly written code (Extended class)to handle click in actionPerformed method), possibly due to waiting time(non-cpu availability at >95% load) and because of this issue, exception is thrown by built in code, which eventually redirects it to error page coded in CustomizedSystemMessages extended class. So all it appears is this error page. No exceptions or error in log file…
If this load is reduced (parallel request)/no of parallel users,load is reduced → failure cases also reduced.

Anyway to get further trace or debug?