As I pointed out at the top of this topic, not much I can do about that. If iTunes reports a 403 (which is an authentication error), I can't really do anything about it since there IS no authentication other than the pairing process.
BTW, I have no doubt that this plugin will eventually stop working. It's based on super old reverse engineered APIs. Apple still has no published API to control iTunes (or music/video as the case may be). When it does stop working, we most likely won't be able to fix it and we'll likely just deprecate the plugin.
There is the iTunes Local Control
plugin in the plugin store that wraps the iTunes AppleScript API in a plugin that will likely suffice for most users, though it does only work with an iTunes instance that runs on the same Mac as the Indigo Server. It does stuff the built-in plugin doesn't do, so you might want to check it out.