Unfortunately, there is no easy way to fix faulty application detection by the Firepower. I have had many instances were legitimate sites were being detected as VPN's or proxies by Firepower. So much so, that we had to remove the application category and only add individual VPN applications that did not cause a problem.
I would create a temporary allow rule for source ip addresses to bypass the VPN block rule. You can add the site URL in the allow ACL conditions. Also, look for the name of the site inside the SSL certificate that it presents, sometimes the Firepower classifies the website based on that information too.
You should open a TAC case and have them open a bug. The more the cases opened for this, the better chance that it gets resolved.
