Blogger: Workaround for Export Blog Failures
I am not a google employee so this is by no means official support and try this out on some test sites first before trying it out on your "production" blog. Searching through the blogger help forum indicated that I was not the only one suffering with an inability to export blog posts and comments. Since I use blogger-in-draft, I don't know if this particular problem exists in regular blogger also but I figured out today that I can get a blog to successfully export by changing the "Convert Line Breaks" option in the "Formatting" options to "No".
Go to your Settings tab, then the Formatting option. Change Convert Line Breaks in the drop-down list from Yes to No. Then export blog as usual.
The help text for this setting indicates that this option controls the behavior of carriage returns in the Post Editor. Setting it to "no" in blogger in draft seems to not have an effect on carriage returns in the new Post Editor because there is a default Post-Option that performs the same function that this global setting enables. This applies only to blogger in draft however--I don't know what impact it will have on the regular post editor but I assume it will remove html line breaks from your posts—which it does not do in blogger-in-draft.
Finally, I don't know why this works. Let me know if it works out for you.