Hi Etienne,
The URL I’m using is correct. It is the same URL that I’m using on my ESXi 6 and 5.5 hosts. https://ESXi_FQDN_or_IP/ui
Are you sure version 5.1 is fully supported?
According to your system requirements “Currently, the host client will only work on ESXi 6.0. When ESXi 5.5u3 is released, it will also work on that version of ESXi.”
The ESXi hosts that I’m testing your application on are all up to date – U3.