Mantis#2027. Thank you kindly, HomerHorwitz for a patch that addresses:

Analysis shows that the XMLRPC-request contains an empty string (<string />) 
for the custom_type parameter, which is deserialized wrongly to a null-value, 
thus leading to the exception above. The attached patch (against r5967) fixes 
the symptom and uses "" for customType in that case.

OpenSim/Data/MySQL/MySQLManager.cs

@@ -902,7 +902,7 @@ namespace OpenSim.Data.MySQL
             parameters["?webLoginKey"] = webLoginKey.ToString();
             parameters["?userFlags"] = userFlags.ToString();
             parameters["?godLevel"] = godLevel.ToString();
-            parameters["?customType"] = customType.ToString();
+            parameters["?customType"] = customType == null ? "" : customType.ToString();
             parameters["?partner"] = partner.ToString();
 
             bool returnval = false;