Could you try disabling XO6?
In a configuration file, enable XO5 as the default version and ensure that XO6 is inaccessible (for example, by using an invalid path or removing the corresponding entry in the configuration). This will help determine if the issue is related to XO6/REST API.
https://docs.xen-orchestra.com/configuration#using-xo-5-as-the-default-interface