"error on line…XML declaration allowed only at the start of the document"

Hello, I've recently installed this theme on my blog and let me tell you that it's one of the best I've ever tried. It's stable, responsive, it fixed my issued with markups...unfortunately though it has somehow broken my sitemap files. In fact, when the theme is activated (and only with this theme) I get this error message:

This page contains the following errors: error on line 1 at column 7: XML declaration allowed only at the start of the document Below is a rendering of the page up to the first error.

...whereas when one of the default themes is activated, the sitemap works just fine. I googled this error and I found out that it's caused by one (or more) whitespace(s) in one of the themes or plugins. Now I'm not sure which whitespaces it's safe to delete and which ones are supposed to be there and I really don't want to make a mess so can someone please help me?