Error:Illegal offset type in isset or empty in wp-includes/post.php on line 1094

Following problem:

I’ve added a CPT and added a single-slug.php to my template – no custom taxonomies, no taxonomies at all. So far everything worked fine for me. When I enter the URL (www.url.com/slug/title/) the template is displayed correctly. But when I try to show all post like (www.url.com/slug/) I’ll get following error:

Warning: Illegal offset type in isset or empty in /path/wp-includes/post.php on line 1094

I tried to ask google but only found websites with same problem ?? Any ideas?

Greets

Greets

https://www.ads-software.com/extend/plugins/types/