Pubclienthttp 601 vs 61

Can someone explain why pub.client.http succeeds in 3 out of the 4 following cases, where one system is running IS 6.01 and the other IS 6.1?

6.01 client, 6.01 server (localhost)–ok
6.01 client, 6.1 server --ok
6.1 client, 6.01 server --ok
6.1 client, 6.1 server – (localhost) Authorization required: invalid session ID or session expired

probably this is the same problem that we observed in 6.01 installation at one of our customers. Once the session expired, a new session id is no longer created which results in this error.
Ask the support if fix IS_6-0-1_SP2_Fix114 is included in 6.1
Regards, Thomas