Blank spaces opening RSS XML file - How to solve?
Some more troubleshooting ideas:


Do the spaces only appear on the RSS page, or also on other pages? That could tell you whether the problem is in a central place or a template/include-file that is only loaded for the RSS feed.
You can use var_dump(get_included_files()) (ideally in the template, just before the XML is generated) to see a list of all PHP files that have been included so far. This might help with investigating - look for closing PHP tags at the end of the files, print statements, etc.


Also, an idea for a dirty workaround (may cause other problems, but could be worth a try if it's urgent): ob_start() buffers all output, while ob_end_clean() deletes it. So if you put an ob_start() in the top of your index.php and an ob_end_clean() at the start of the RSS template, this might let you discard the spaces. (It shouldn't affect anything else, because the buffer is flushed automatically if it's still open when the script ends.)