I figured it out, the problem was indeed in the html. The problem arises when you want to have a java popup of a certain link. So as an folder administrator I insert a link via the “advanced editor” and directly add a java popup of the link . The resulting html code looks like this:

<a onclick="window.open('http://example);return false;" href="http://example">example</a></span></td>

I can save it and the code is executed. When I now change to a user with editor rights, I can open up the editor but as soon as I change some of the information on the page and want to change it I get the error:

Unable to save changes due to the following validation errors:
•    Illegal attribute 'onclick' on element <a>.

When I remove the javascript popup via code or the advanced editor the error does not appear and I can save and edit the wikipage with that non-administrator user. In order to circumvent this I can just use rel="noopener noreferrer" target="_blank" in order to send the link to a new tab without the javascript popup.

What I think is still interesting that as an full site administrator it is possible to implement the <a onclick> while as an “folder administrator” or “editor” this line of code results in a validation error.

Thank again for the help, you are awesome as usuall!
Greetings from Germany,
Max