Getting Error on checkout page

Yesterday I experimented with some tax settings and installed an xml file which adds all countries to geozones, and I enabled seo urls.

So today I decided to test my website, everything works fine but something goes wrong on the checkout page.
When I select a billing address (existing address) and click on continue I'm getting an alertbox with "Error" in it.

There is no further information about the actual error, but when I look at the error log in my admin I see the following:
I uninstalled the xml which I added yesterday, but it doesn't solve the problem. My guess it has something to do with either the tax settings or my seo url setting. How can I solve this problem ?

i have the same problem with 1.5.6 version. For default theme and fresh install.