How do you set the property flowable.modeler.app.deployment-api-url? Do you use placeholders for the hostname and the port? Because it seems they will not properly resolved.
Are you sure you don’t have any other property with the same value, overwriting your setting? The fact the url is changed to ‘http://:/flowable-task/…’ is really weird, the value of the property is taken as-is and then values are appened.