Is the first image the new behaviour? If so it looks like there is probably a fatal php error. Can you turn your Joomla debug to maximum and turn on the debug joomla option in the admin and see what error is reported,

yes, the first image showed the new behavior. turning on Joomla debug to maximum unfortunaltely shows no error report information. The page looks the same. (Screenshot with Source-Code attached as image.

++++++++++++++
<!-- Event Calendar and Lists Powered by JEvents //-->
<div id="jevents">
<div class="contentpaneopen jeventpageyellow jevbootstrap"
id="jevents_header">
<h1>
Veranstaltungen </h1>
<ul class="actions">
<li class="print-icon">
<a href="javascript:void(0);" rel="nofollow"
onclick="window.open('/cms/index.php/veranstaltungen/cat.listevents/2022/09/15/-?print=1&pop=1&tmpl=component', 'win2', 'status=no,toolbar=no,scrollbars=yes,titlebar=no,menubar=no,resizable=yes,width=600,height=600,directories=no,location=no');"
title="Drucken">
<span class="icon-print"> </span>
</a>
</li> <li class="email-icon">
<a href="javascript:void(0);" rel="nofollow"
onclick="window.open('/cms/index.php/component/mailto/?tmpl=component&link=92a3ff4972fc93e21178468160f850c6a372b5ab','emailwin','width=400,height=350,menubar=yes,resizable=yes'); return false;"
title="EMAIL">
<span class="icon-envelope"> </span>
</a>
</li>
</ul>
</div>
<div class="contentpaneopen jeventpageyellow jevbootstrap" id="jevents_body">
<div class='jev_pretoolbar'></div>
++++++++++++++++

Do you show any javascript errors in the developers console of your browser?

Is there any chance that you have a template override for the navigation toolbar?

Also it looks as though your server doesn't output error messages to the browser - can you check for an error log file in the root folder of your site or check the error logs at your hosting company.

No there are no javascript errors in the console (s screenshot)

Is there any chance that you have a template override for the navigation toolbar?

Also it looks as though your server doesn't output error messages to the browser - can you check for an error log file in the root folder of your site or check the error logs at your hosting company.

the server outputs error messages to the browser - i checked the error log file - there are no entries - neither in the joomla-error log nor in the apache-server log.

Concerning navigation toolbar override - i will check!

Concerning navigation toolbar override - i will check!

Did you figure this out? Looking at your screenshot it looks as though you have had some custom coding done on this in the past - this is probably where the error is