Error when generating the sitemap
There are probably empty lines in one of these files
In wp-config.php (WordPress) there must be no blank lines before the <?php or after the ?>.
Also in the Functions.php of the WordPress theme, there must be no blank lines before the <?php or after the ?>.
This causes 2 blank lines to appear before the <!DOCTYPE html>, resulting in an error message
Have a look at the source code of your page
You use Yoast anyways, why you have a second xml creation plugin from Bestwesoft?
Could be a conflict.
Also where is the forum?