Do you mean the SSLVPN connected just fine when a licensed FCT is installed?
Kindly check if your FGT is configured to allow only EMS-joined FCT to connect SSLVPN:
Is this issue OS-specific? (happens only to Windows or macOS)?
We have to narrow down whether the issue is in FGT side or endpoint side.
It is best to troubleshoot via a FortiCare ticket with your FGT S/N or EMS S/N.