Conflict between Yoast and thirstyaffiliates

@lipsum We’re sorry to hear you are experiencing this issue on your site.

The error ‘XML declaration allowed only at the start of the document’ is caused when another file, plugin, or theme adds white space to the beginning of the sitemap file. You will need to view the source code of the sitemap_index.xml document in order to confirm this (see image here). We also have additional information on this and how to resolve this error at the following link: https://yoast.com/help/xml-sitemap-error/.

In order to resolve this, we would first recommend clearing your site’s caching. If you are not sure how to clear your server caching, please speak with your host provider. Does clearing your caching from your caching plugin (W3TC, WP Supercache, etc), server (Varnish, LiteSpeed, or others), or CDN (such as Cloudflare, Amazon AWS, etc) resolve the issue? If after clearing your site’s cache you are still experiencing the error on the sitemap, we would then next recommend checking your theme’s functions.php file for an empty line at the end of the file, as well as your wp-config.php file for any additional spaces at the beginning of the file.

If you cannot find any empty lines at the end of your theme’s functions.php file or blank lines or spaces at the start of the wp-config.php, we would next recommend running a conflict check to identify whether another plugin or them is outputting the additional white space in the sitemap file. The fastest way to do this is to deactivate all non-Yoast plugins and switch to a standard theme like TwentyTwentyOne.

Test this on your development or staging site, if you have one. If not, we recommend using the Health Check & Troubleshooting plugin. This plugin allows you to run a conflict check without affecting normal visitors to your site. If you are unfamiliar with checking for conflicts, you can find a step-by-step guide that will walk you through the process here: How to check for plugin conflicts.

Can you please try the aforementioned suggestions and let us know whether this resolves your issue?

@lipsum

In our local testing environment with only TwentyTwenty default theme, Yoast SEO free and Thirstyaffiliates plugin enabled, we tried to reproduce the given XML error on the sitemap but to no avail.

You’ve mentioned the error does not come up with the Thirstyaffiliates Pro plugin. We are not sure whether issue occurs in combination with both of those plugins activated, please make sure that your website meets the plugin requirements.