While working on a recent project work using CRM 2011 at a client location, I saw the below error in the log file:
Crm Exception: Message: Expected non-empty string., ErrorCode: -2147220989, InnerException: System.ArgumentException: Expected non-empty string. Parameter name: userPrincipalName
On further investigation I found that a rule is needed in the Claims Provider Trusts for Active Directory as shown in the below screenshot.
After I created this rule, the error stopped appearing in the log file and the issue was fixed.