In the New Credentials dialog, if a user puts a space before or after the 
Username (most likely by mistake or because of a cut-and-paste), the 
Username will be considered invalid:

   Krb5: Client not found in Kerberos database

Would it be possible to pre-validate the Username to make sure it doesn't 
begin/end with spaces?   Then either explicitly say the Username shouldn't 
begin/end with spaces or just silently strip them.

Along the same lines, if the Realm begins/ends with a space the user will 
get the following message:

   Krb5: Cannot resolve network address for KDC in requested realm

Same thing here... report an error or strip the leading/trailing spaces.