Unfortunately something changed recently with Blogger and auto-configuration is no longer working as expected. I am working on a fix for the next update to MarsEdit, but in the mean time you can manually fix the settings in MarsEdit to address the problem:
To open the blog settings in MarsEdit, open the Main Window, and find your blog in the list on the left side of the window. Double-click the blog icon to open the settings window. Then change the "API Endpoint URL" setting and "Blog ID" setting for your blog to:
API Endpoint URL: http://www.blogger.com/feeds/[YourBlogID]/posts/default
Blog ID: [YourBlogID]
In this case because your blog has already been setup with "the wrong" ID, the [YourBlogID] part in the settings above should be the numeric part BEFORE the & in the existing setting. E.g. if the blog settings were currently:http://www.blogger.com/feeds/12345&zx=2354aa7f-a08a-482f-bcc5-ad78f0934e8e/posts/default
Then you would want to change them to just:http://www.blogger.com/feeds/12345/posts/default
(Obviously, make sure to use the right number for [YourBlogID]).
Click OK and then "Refresh" the blog. It should work correctly now.