We are not able to replicate this issue locally. Can you reproduce it on our demo website
If you use Active Directory
where the users are identified by SID you may get this error. Sitefinity uses Guid and the newsletter expects Guid instead of SID. If this is the case you need to convert SID to GUID or use Guids with AD. You should override the membership provider classes and make a Guid - SID mapping. By default we call GetUser by ProviderUserKey
// check if providerUserKey is Guid and get its mapped SID
the Telerik team
Do you want to have your say when we set our development plans?
Do you want to know when a feature you care about is added or when a bug fixed?
Telerik Public Issue Tracking
system and vote to affect the priority of the items