1: In order for Exchange integration to work with Lync or Skype for Business your Exchange email address and password must match your Lync or Skype for Business username and password.
2: Exchange auto discover records must also be in place and fully functional
If the 2 prerequisites are met then to resolve this issue you will need to uninstall and reinstall the Lync mobile app (see steps below).
Step 1: Open the Google Play Store
Step 2: Swipe to the right from the far left side of the screen and select "My apps"
Step 3: Find and select the Lync 2013 app
Step 4: Select the option to uninstall
Step 5: When the uninstall is completeled you will see the option to "Install" reappear
Step 6: Install the Lync 2013 app and sign in using your normal username and password