There is the solution.
I opened a ticket at IronPort support. After some investigation the problem was found.
Following the official statement:
We identified an issue with AsyncOS for Web 7.1.0-306 where under certain circumstances under high load, the box can stop responding to requests requiring a reboot.
To avoid service interruptions we released a fix for this issue and we suggest all customers running 7.1.0-306 to upgrade to the new 7.1.0-307 build.
Thanks to IronPort customer support!