error connection when connecting to website

Our user need to access one portal for procurement.

some times users will encounter issue accessing the URL. error message is ERR_CONNECTION_CLOSED.

we have whitelist the URL so that users wun get blocked

what else we can do to ensure users don't get this error_connection_closed error ?

what our users noticed is if they launch another new session using different browser there will be no issue but will get the same error if launch using same browser.