Symptoms

In the ADUM logs the client saw the following error being generated:
"2016-04-07 13:11:49::939", "Error", "GetUser:MemberOf", "No mapping between account names and security IDs was done", " at ADUM.Translate.DNToName(String DN)
at ADUM.K2UserManager2.GetUser(String Name)", "Additional Information: ", "MemberOf Resolve(CN==Distribution_List],OU==Group],DC==com])"
 

Diagnoses

We enabled full K2 logging and saw the following error being thrown at the same timestamp the ADUM logs were thrown:

"2016-04-12 17:34:56::767", "Debug", "GetUser", "User rDomain]nUsername] does have a memberOf property."
"2016-04-12 17:34:57::095", "Error", "GetUser:MemberOf", "No mapping between account names and security IDs was done", " at ADUM.Translate.DNToName(String DN)
at ADUM.K2UserManager2.GetUser(String Name)", "Additional Information: ", "MemberOf Resolve(CN=NDistribution_List],OU=UGroup],DC=Ccom])")"

The Group that the error is referring to is a test group that was deleted, and no longer in use. Using Active Directory we can see that the Group did not show.

The client downloaded the following tool:
https://technet.microsoft.com/en-us/sysinternals/adexplorer.aspx

Using the AD Explorer tool we could clearly see a link to the Group on the User
 

Resolution

The issue was solved internally by deleting the Group using the AD Explorer Tool