How long after you started the extension are you trying to access it? Sometimes it may take a few minutes for them to be ready. If it has been up for some time you can try to reset it and recreate the image.
diagnose docker reset <container extension>
You can also check that you have the latest docker images:
diagnose docker upgrade <docker extension>
what kind of resources do you currently have assigned to your Fortimanger VM in regard to CPU/memory?
The Fortinet Security Fabric brings together the concepts of convergence and consolidation to provide comprehensive cybersecurity protection for all users, devices, and applications and across all network edges.