As you mention endpoints have to be correct. Check if you have more than one binding in IIS for that site try to remove everyone except one and check endpoints in web.config making sure you are using the one that the site is binding to in IIS. I have had the same issue and i did that plus i reinstalled livemonitor and it worked after that.
I am running into a strange issue with CMO live monitor - it comes up with 'error-failed to load root node' . But here is the background what is happening behnind the scenes.
1) It works fine when accessing the site with its internal IP - no isues here.
2)But when access the site with external IP / URL the live monitor fails to load, I've correctly configured the end point addresses as describled in various other posts with similar error, and I can browse the TraceService with the external URL . However fiddler shows that requests going through crossdomain.xml and clientaccesspolicy.xml - I've also tried drop these files with contents from msdn and then it just stuck at these two files. Just to add the external URL comes through a load balancer. but end point configuration looks ok to me as the service itself loads and there are no errors / exceptions anywhere in logs or fiddler.
Please let me know if anyone has any idea why its happening and how to resolve this.