After troubleshooting for several hours earlier today I was getting 500 errors when trying to publish to my WordPress blog via Live Writer or Word 2010. Would get a standard WordPress xmlrpc 500 error trying to hit xmlrpc.php but the thing was I can hit the page directly and it was working for my other blogs which were upgraded to WordPress 3.5.1 the same day.
Naturally whenever you are troubleshooting a WordPress problem the first thing you need to do is disable all plugins and re-test to see if same results occur, more often than not a plugin conflict is the cause of many WordPress issues.
I disabled all 48 plugins on DragonBlogger.com and one by one enabled them and re-tested, and I successfully narrowed down the problem to the Peter’s Collaboration Emails plugin version 1.9 which has a problem with WordPress 3.5.1 and causes the WordPress XMLRPC 500 error.
I confirmed by activating, reproducing the error and deactivating the plugin a few times to 100% narrow it down. I have left a message in the forum for the plugin to inform them of the issue so that hopefully it will be resolved soon.
Peter’s Collaboration Email is a fantastic plugin that emails the WordPress blog owner when a guest post is submitted for review, it also notifies the post author whenever there is a status change such as when you schedule or publish it so they know automatically when their blog post is scheduled to go live. So I hope they fix the plugin soon and this because a short term issue.
Peter’s Collaboration Email plugin version 1.9.1 was just released which resolves this issue, so as long as you update to 1.9.1 and later then you should have this plugin as the cause for XMLRPC 500 errors with WordPress anymore.