sitemap.xml with XML Parsing Error

Hi all,
I registered some days ago that there is a Problem with Google XML Sitemaps Plugin on my page while loading sitemap.xml.

I got following failure, but was until now unable to fix this (1. Deactivate plugin. 2. Remove and install plugin again 3. deactivate specific .htaccess and WP hardening options)

Google XML Sitemaps Plugin Version: 4.0.9

XML Parsing Error: XML or text declaration not at start of entity
Location: https://www.zenerino.ch/sitemap.xml
Line Number 2, Column 1:<?xml version="1.0" encoding="UTF-8"?><?xml-stylesheet type="text/xsl" href="https://www.zenerino.ch/wp-content/plugins/google-sitemap-generator/sitemap.xsl"?><!-- sitemap-generator-url="https://www.arnebrachhold.de" sitemap-generator-version="4.0.9" -->

On my test site the plugin is running well and generating the xml site as expected. I just got on the production site this failure. I use the plugin Autoptimize. Could this maybe generate this error?
Does anyone have an idea how to solve this?

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