The tasks given by the REST task endpoint present a wrong url in taskInfo/taskActions.
For example :
http://localhost:8080/nuxeo/site/api/v1/task41866d44-7b02-4331-bf0a-7479776fddda/validate
The correct url should be :
http://localhost:8080/nuxeo/site/api/v1/task/41866d44-7b02-4331-bf0a-7479776fddda/validate