FortiClient VPN connection error (-14)

Hi,

A user is trying to set up a connection through FortiClient. He has MFA enabled. When he connects and approves the MFA notification, he gets the following error:

"Unable to establish the VPN connection. The VPN server may be unreachable. (-14)"

We've tried many default fix options already, but unfortunately it doesn't work.
This user is member of a security group X. When I added the account to the security group that admins are member of, and then set up a connection, it can connect. When I then remove it from that security group again it gets the error again.

I exported a log from the FortiClient, but it only says:  msg="SSLVPN tunnel connection failed"

Does anyone know what the issue can be here?

Thanks in advance.