Hi and welcome to Digi Forum.
In order to investigate this issue, some more details would be needed regarding how is the EX15 configured and the complete setup.
I would suggest you to send us an email to email@example.com in order for us to look at it. Please include a support report from the device when the issue occurs (https://www.digi.com/resources/documentation/digidocs/90002344/default.htm#os/support-download-report.htm?TocPath=Diagnostics%257C_____1).
Please also note, that if your company has a Base support level, the help we can offer on VPN troubleshooting is limited.
Please check here more details about Digi Support options: https://www.digi.com/support
Digi Tech Support