Solved: reason for the problem is an active function of WP plugin W3 Total Cache (Version 2.1.1).
In General Settings of the plugin in the section User Experience
the function Lazy Load Images (Defer loading offscreen images)
must be deactivated to solve the problem.
Thanks for responding
I will sent you infos and link to your email address (joedolsom.com).
Sorry forgot escaping code afterclick:
Code after click:
<span class="category-color-sample">
<img class="lazy" src="data:image/svg+xml,%3Csvg%20xmlns='http://www.w3.org/2000/svg'%20viewBox='0%200%201%201'%3E%3C/svg%3E" data-src="https://gewo-rt-intern.org/wp-content/plugins/my-calendar/images/icons/star.png" alt="" style="background:#0b4967;">
</span>
