Python App Engine's urllib2: works locally but not when deployed to GAE
We make no guarantees that urlfetch calls will all go out on the same IP address.
I have an App that worked well on both GAE and test server till a few days ago. It connects to a remote site, logs in and browse pages and input information automatically. The remote site is using dynamic URLs to follow the session, each page gives the link for next call.
The program is very basic : urllib2.urlopen then regexp to extract the next url key then new call to urllib2.urlopen and so on.
Now my app works still perfectly on test server but fails when deployed on GAE : I have a series of calls to urllib2.open and most of the time, the remote site says it has lost the session already on the second call but 1/10th I could go to the third call and once GAE has gone successfully to the fourth call. 
This seems to point out that it is not a security issue with the remote site (which has not changed) nor a question of redirect and cookies I have read in other posts.
Users reported to me that it worked well till the 14th of Sept 13 and the failure was reported to me on the 20th. Was there a change in the handling of URLfetch in GAE recently? 
I've just spent 2 days on the problem with no tangible clue. 
It may be a question of IP address? The remote server could control the session with the IP adress and the dynamicURL together and I can imagine that GAE does not garantee that in a same call to GAE, all calls to URLlib are handled by the same machine? This could explain why sometimes it works for two or three successive URLs. I do not know enough GAE internal mechanism to confirm.
Thank you in advance for your ideas.