MantisBT 1.3.x disables inline javascript. The plugin needs to be revised to move the javascript to a .js file and remove the php calls from within the javascript code to get the OAuth app information from the plugin configuration.
Check #6 . I've made some update to put javascript in a separate file and load config in meta tags in header of page.
It works on my installation (2.6.0)
MantisBT 1.3.x disables inline javascript. The plugin needs to be revised to move the javascript to a .js file and remove the php calls from within the javascript code to get the OAuth app information from the plugin configuration.