Cant open a technical case

Hi guys,

i am getting this error while trying to open a technical support case. I am filling all required data with a valid support contract 

CaseOwner_AutoFollowUnFollowCase: execution of AfterInsert caused by: System.DmlException: Insert failed. First exception on row 0; first error: INVALID_CROSS_REFERENCE_KEY, This user cannot follow any other users or records: [] Trigger.CaseOwner_AutoFollowUnFollowCase: line 103, column 1

any idea?