I saw this exact issue in v4.4.0 and I believe it was fixed in the now obsolete 4.4.1 release. I had a ticket (16703) related to this issue. I have not seen this issue since.
Have you read this article?
http://www.mendix.com/tech-blog/take-a-time-out-to-learn-about-asynchronous-calling/
I think this explains the issue. Different users will be behind different firewalls or proxy servers with different time-out settings, so this would explain why you can't reproduce the issue yourself.