Hey Patryk,
You should post to the
Public Issue Tracker, including all relevant code snippets, the full stack trace, etc. We monitor there all day and will pick up the issue relatively quickly. It's hard for me to speak on the specifics of what might be behind this issue without that information. Generally, the App Engine Google Groups forum isn't the best place to post platform issues like this, since this forum is meant for more high level and general discussion of the platform and services, patterns people use, etc. so I recommend posting to the Public Issue Tracker.
Nick
Cloud Platform Community Support