The event text/description is not displaying on every page

There is very little I can analyze from what you say, as if description displays on one post and not another then that has to be external to the plugin as the plugin has no logic to differentiate.

The code is simply

$output .= apply_filters( 'qem_description', get_the_content() );

What can be happening is some custom code is either filtering qem_description or something is impacting the WordPress query responses.

As this may be a plugin or theme conflict. Please attempt to disable all plugins, and use one of the default (Twenty*) themes. If the problem goes away, enable them one by one to identify the source of your troubles.

If you can install plugins, install and activate “Health Check”: https://www.ads-software.com/plugins/health-check/
It will add some additional features under the menu item under Tools > Site Health.
On its troubleshooting tab, you can Enable Troubleshooting Mode. This will disable all plugins, switch to a standard WordPress theme (if available), allow you to turn your plugins on and off and switch between themes, without affecting normal visitors to your site. This allows you to test for various compatibility issues.
There’s a more detailed description about how to use the Health Check plugin and its Troubleshooting Mode at https://make.www.ads-software.com/support/handbook/appendix/troubleshooting-using-the-health-check/

He,

I have performed the checks, but the issue persists. All new events that I post are displayed without a description. On https://landgoedsteenbergerveld.nl/event/wordt-verwacht-leoni-jansen-solo-concert/, you can see an older event where the description is displayed.

what else can i do?

I really can’t think how that can happen, i.e. works on some but not others.

If you screen shot ALL your Quick Event Manager settings tabs , the full edit event page for one that works and one that doesn’t, and also provide Tools > Site Health – Info Tab – <Copy site info to clipboard> button and paste that too, we will see if we can replicate the issue in test.

Please remember to obliterate anything in your screen shots that reveal email addresses as this is a public form.