trying to issue security token: The security token username and password could not be validated..

in SharePoint 2013 portal we have customized FBA in PROD (2xWFR , 2xApp, Clustered DB SQL 2014) and when users try to login below exception caught on ULS and as we have 400K users we are getting hundreds of this errors , any help why this happening will be appreciated

- SPSecurityContext: Request for security token failed with exception: System.ServiceModel.FaultException: The security token username and password could not be validated.    
 at Microsoft.IdentityModel.Protocols.WSTrust.WSTrustChannel.ReadResponse(Message response)    
 at Microsoft.IdentityModel.Protocols.WSTrust.WSTrustChannel.Issue(RequestSecurityToken rst, RequestSecurityTokenResponse& rstr)    
 at Microsoft.IdentityModel.Protocols.WSTrust.WSTrustChannel.Issue(RequestSecurityToken rst)    
 at Microsoft.SharePoint.SPSecurityContext.SecurityTokenForContext(Uri context, Boolean bearerToken, SecurityToken onBehalfOf, SecurityToken actAs, SecurityToken delegateTo, SPRequestSecurityTokenProperties properties)


An exception occurred when trying to issue security token: The security token username and password could not be validated..

Token Cache: Failed to rebuild forms user token for user '0#.f|SISmembershipprovider|1048470403'.
Token Cache: Failed handle missing token for user '0#.f|SISmembershipprovider|1048470403'.