This is an extremely unlikely scenario as ExpressVPN is constantly maintaining its servers. Plus I’ve used ExpressVPN daily for nearly a year, and I’ve never encountered any server errors.
But if you do happen to connect to a server that doesn’t work, I recommend simply switching servers. It only takes 2 clicks to change to a new server.
If you’re looking to connect to a specific server that’s down, then I suggest contacting the customer support team over 24/7 live chat instead. It’s a quick and easy way to get a list of updated servers.