Kerberos Authentication problem

Hi everyone,

i have a problem with my kerberosauthentication wiht explicit proxy Fortios 5.6.5.

Most times it works fine when a user authenticate but sometimes users dont come with the right name to the fortigate.

I can see in the Firewall user monitor that some usersnames are clientname$@REALM and they wont work.

I dont get it why the dont come with Username@REALM. 

Can someone help me please

Thank you