I’m sorry but I can’t think of how Display Posts would interfere with your menu. Display Posts loads no CSS or JavaScript, so there’s nothing in the plugin that could conflict with your theme’s menu.
I recommend you use the Health Check plugin to further test the issue as it’s likely a different plugin causing the issue: https://displayposts.com/2019/04/20/using-health-check-plugin-to-find-theme-and-plugin-conflicts/
Thanks for the reply.
I checked the Plugin before, but did not find anything.
Reinstalled WordPress, Plugins, Theme and custom code. It is working now. I did not find any errors while checking everyhting. I still don’t know what caused the error and why disabling the plugin worked.
Resolved,
Cheers!