AuthenticationException when running proxy inside IIS

[enkelmedia]

I have a .NET 5 app hosted inside IIS, I'm using Selenium to perform fetch information from websites and use the Titanium Proxy to intercept the requests.

It all works fine on my dev-machine but on Windows 2019 server we're getting errors like this:

Titanium.Web.Proxy.Exceptions.ProxyConnectException: Couldn't authenticate host 'accounts.google.com' with certificate '*.google.com'.
 ---> System.Security.Authentication.AuthenticationException: Authentication failed, see inner exception.
 ---> System.ComponentModel.Win32Exception (0x80090327): An unknown error occurred while processing the certificate.
   --- End of inner exception stack trace ---
   at System.Net.Security.SslStream.ForceAuthenticationAsync[TIOAdapter](TIOAdapter adapter, Boolean receiveFirst, Byte[] reAuthenticationData, Boolean isApm)
   at Titanium.Web.Proxy.ProxyServer.handleClient(ExplicitProxyEndPoint endPoint, TcpClientConnection clientConnection)
   --- End of inner exception stack trace ---
   at Titanium.Web.Proxy.ProxyServer.handleClient(ExplicitProxyEndPoint endPoint, TcpClientConnection clientConnection)

We've tried to figure out what's wrong but without any luck. It feels related to certificates and/or user permissions. It's interesting that when we tried to set the App Pool identity to "Administrator" it worked, but not when using AppPoolIdentity or NetworkService.

Any ideas?