majbom

https://kb.yoast.com/article/58-xml-sitemap-error

– try disabling other plugins to see if some of them are making whitespaces

I’ve also just updated (same versions as OP, and get this warning, when debug is on:

The called constructor method for WP_Widget is <strong>deprecated</strong> since version 4.3! Use __construct() instead.

No one?

Found a solution yet? I have the same problem on several sites

Found out that php5-json not is installed by default on ubuntu 13.10

sudo apt-get install php5-json

made the site work perfectly!