Event ID 364- how do i troubleshoot this?

I keep getting the following error on both of our ADFS proxy servers.

Our setup is the following,

2 NLB ADFS Proxy servers, with 2 ADFS servers with there own NLB. These errors occur regulary throughout the day but the setup seems to be working fine from an end user perspective.

Encountered error during federation passive request.

Additional Data

Exception details:
Microsoft.IdentityServer.Web.AuthenticationFailedException: MSIS8108: Authentication failed.
   at Microsoft.IdentityServer.Web.FederationPassiveAuthentication.SubmitRequest(MSISRequestSecurityToken request)
   at Microsoft.IdentityServer.Web.FederationPassiveAuthentication.RequestBearerToken(MSISSignInRequestMessage signInRequest, SecurityTokenElement onBehalfOf, SecurityToken primaryAuthToken, String desiredTokenType, Uri& replyTo)
   at Microsoft.IdentityServer.Web.FederationPassiveAuthentication.BuildSignInResponseCoreWithSecurityToken(SecurityToken securityToken, WSFederationMessage incomingMessage)
   at Microsoft.IdentityServer.Web.FederationPassiveAuthentication.BuildSignInResponseForProtocolRequest(FederationPassiveContext federationPassiveContext, SecurityToken securityToken)
   at Microsoft.IdentityServer.Web.FederationPassiveAuthentication.BuildSignInResponse(SecurityToken securityToken)