When an IPsec VPN profile with a pre-shared secret is configured on an iOS device, the device would initially prompt for the password with a connection is attempted. When the password is entered, VPN connects successfully. However, after a few hours when VPN connection is attempted, the device stops prompting the user for a password and just throws the “User authentication failed” message.
We have informed Apple of the issue and this article will be updated once we confirm a resolution in iOS. If you have Apple Care Support you may wish to reach out directly to Apple to learn more details around the status of this issue.