webcheck

Same error on my site after upgrading.

Dirty and quick fix: Download xml-sitemap.xsl (from the folder /wp-content/plugins/wordpress-seo/css/) and reupload this file as sitemap.xsl to your home folder.

So to be on the same track: We are all talking about an unwanted output before the xml tags start, so the sitemap will fail to parse.

Possible problems as discussed so far:
1) WordPress 3.6
2) Plugin incompatibility
3) Size malfunction
4) Execution time malfunction

Lets rule some of these out.
1) and 2) might not be the problem – I got the error running 3.5. Transferring all the data to another webhoster couldnt reproduce the error (remember: Transferring ALL data – plugins, theme, database, etc).
So 3) wont be the problem neither – because the error would have come up on the other webhost aswell.
4) reducing the amount of data to process (and therefore reducing execution time) solved the case for me. For others it didnt – so this might be one trigger, but its very likely that there are others involved too.

Another guess: Did any of you manually import data into the wordpress database?
After I did, the error showed for the first time. After manually checking my database entries I found a strangely formatted entry, rewrote this – and got still an error (but a different output before the xml).
Perhaps its a combination of those two factors: Execution time, and wrongly formatted entries?

When the error occured on my website, WordPress 3.5 was up and running. So I’m still tending to point at the execution time.

Perhaps Yoast is capturing the entries for the sitemap in a different, far more execution time expensive way than the other plugins?

@crashguru: Did you try my suggestion reducing the number of entries in Yoast? I’m still pretty convinced of this as a possible solution.

Perhaps an even lower value is needed for your hoster? Just for fun try setting it to 5. If its working, try bumping this up step by step. If its still broken I withdraw my workaround.

Found a possible workaround: Limit the number of entries displayed per sitemap-page. You can adjust this via Yoast -> SEO -> XML-Sitemap and enter a number in the field at the bottom. 25 worked for me.

A recap of my problem: Ran into the same as Tony. Only page-sitemap.xml echoed this strange error (or more explicit: a leading number, so that the xml would fail to parse).
Due to this limited error, I suspected the page-content to be faulty. After some checks I was able to rule this out. So I exported the whole content and uploaded it to another of my test system (with another hoster): Et voila, everything was perfectly fine.
So my conclusion: I guess the max_execution_time lead to this error. On my live system (where the error occured) the max_exec. is 30. On my test server 60.

So limiting the displayed items solved the problem for me. Please try this yourself and give some feedback to my workaround.