sitemap fail

Dear AioSEO Team!

I tried to use your excellent plugin, but I had a bit of problem whit the sitemap function.

When i made my settings, the URL/sitemap.xml had this error message:
“This page contains the following errors:
error on line 2 at column 6: XML declaration allowed only at the start of the document
Below is a rendering of the page up to the first error.”

The sitemap.xml source:

<?xml version="1.0" encoding="UTF-8"?>
<!-- Sitemap dynamically generated by All in One SEO Pack 2.3.16 by Michael Torbert of Semper Fi Web Design on Fri, 25 Aug 2017 11:41:50 UTC -->
<?xml-stylesheet type="text/xsl" href="https://domain/sitemap.xsl"?>
<sitemapindex xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
	<sitemap>
		<loc>https://domain/sitemap_addl.xml</loc>
	</sitemap>
	<sitemap>
		<loc>https://domain/sitemap_page.xml</loc>
	</sitemap>
	<sitemap>
....and more

AND, if I disable “Dynamically Generate Sitemap” option, the sitemap cannot give me error messages, just a white blankpage.

What cause this issue?
Do you have any solution for this problem?
What is the recommended server-settings, or anything else?

The ChromeConsole says these:

- XML declaration allowed only at the start of the document
- Resource interpreted as Stylesheet but transferred with MIME type text/xml: domain/sitemap.xml

Manythanks!