a misleading "Access Denied" error when joining a existing sharepoint farm

Lately I have tried to join a new Report Server to an existing sharepoint 2007 farm as it is required for SSRS in integrated mode, but the configuration wizard fails at the step 2, i,e, connecting to configuration database, and I got the following error from window event log:

Failed to connect to the configuration database.
An exception of type System.Security.SecurityException was thrown. Additional exception information: Access denied.
System.Security.SecurityException: Access denied.
at Microsoft.SharePoint.Administration.SPPersistedObject.Update()
at Microsoft.SharePoint.Administration.SPServer.Update()
at Microsoft.SharePoint.Administration.SPFarm.Join()

As the error message indicates, all my thoughts are on the database connection, for that, I first check if the sql server is ping-able, and then the permission on config database, and then whether window firewall is open etc... everything is fine, I can even create a new farm with the same sql server, but I could not join!

At last, i found i have it is caused by different sharepoint versions: one with April 2010 CU and the other is just SP2. This is nothing to do with database. After installing the CU, the error disappears, and join is successful. I hope this can save me or someone else several frustrating hours next time.