We have the same error since update to 9.1R15. The interruptions only happen in ESP mode - mostly around the time of a Key Exchange. Many users have interruptions after 15 to 2500 seconds, mostly around 500 - 800 seconds. Some have to redial, others don't notice it - depending on how long the interruption lasts (> 4-6 seconds mostly redial). For some users working with WLAN, there are entries in the Windows event log that WLAN security has been terminated and that the WLAN connection will be disconnected. Whether this is a consequence of the VPN interruption, or whether the VPN connection is disconnected due to the termination of the WLAN security, I could not determine yet. It is often difficult to isolate the problem because the users have little PC experience and I cannot access the client logs. SSL connections run without errors.
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