Attempt to fix mantis issue # 65, seems like it is a race condition between two regions trying to add a user to the AssetTransactionManager at the same time. So have placed a lock around the Dictionary add.

OpenSim/Framework/Communications/Cache/AssetTransactionManager.cs

@@ -48,6 +48,8 @@ namespace OpenSim.Framework.Communications.Cache
 
         // Methods
         public AgentAssetTransactions AddUser(LLUUID userID)
+        {
+            lock (AgentTransactions)
             {
                 if (!AgentTransactions.ContainsKey(userID))
                 {
@@ -55,6 +57,7 @@ namespace OpenSim.Framework.Communications.Cache
                     AgentTransactions.Add(userID, transactions);
                     return transactions;
                 }
+            }
             return null;
         }