Exchange 2013 CAS

Cuurent scenario Exchange 2013 installed on Windows 2012 environment consist of two CAS in windows NLB and Two mailbox server.

Problem statement:When users are accessing the owa with owa url they getting passwrd prompte time again. we tried with IP address of one of the cas server users authenticated. when we try with another CAS serve IP we geting password promting time and again.

With owa url which is pointing to nlb VIP. when owa accessed by owa url from internet in some clients we are getting below error

Something went wrong

A mailbox couldn't be found for Domain\usermailbox If the problem continues, contact your help desk.

X-OWA-Error: Microsoft.Exchange.Data.Storage.AdUserNotFoundException

X-OWA-Version: 15.0.775.32
Verified both CAS all virtual directory all settings intact. the Setup was working perfectly suddenly it is satarted.
Please suggest.