warning message '[function.symlink]: File exists'

@joachim: You should switch of error output to the page in admin > site configuration > error reporting.

However, I'm also getting this error in the drupal php log

Perhaps a check is need to ensure the link doesn't exist first before creating it?

e.g

<?php
/* boost.helpers.inc line: 83 */
if(!file_exists(implode('/', $link))){ 
  /* make the link */
  return symlink(implode('/', $target), implode('/', $link));
}
?>

would that suffice?

or maybe:

<?php
/* boost.helpers.inc line: 83 */
if(file_exists(implode('/', $link))){ 
  /* delete the existing link */
  unlink( implode('/', $link) ); 
  /* make the new link */
  return symlink(implode('/', $target), implode('/', $link));
}else{
  /* link doesn't exist, just create it */
   return symlink(implode('/', $target), implode('/', $link));
}
?>

DT

This issue removes all symlinks; merging all symlink issues into this one.
#174380: Remove symlink creation. Let each path have own file