I'm using Jsitemap and SH404sef, and found that the XML sitemap was rendering as HTML rather than XML.
To fix this, go to the configuration options in SH404SEF, General > Advanced tabs and ensure that the setting '301 redirects from non-SEF to SEF' in the configuration of sh404sef is not enabled.
If you still experience issues after changing this setting you can also change the sh404sef settings: configuration > by component and choose 'Leave as non-sef' or 'Use Joomla router' for Jmap.
Additionally, within the Jsitemap settings Configuration > Advanced settings it's important to ensure that the following parameter is disabled and set to 'No' as by default in order to always use native raw links for each sitemap: