500 server error on single listing pages in multisite

Good morning,

we have purchased the Directory Theme for a multisite project and are now expierencing some troubles with the listings: When we try to open a single listing entry the page doesn’t load and a 500 server error occurs. This only happens on the listing entries. The rest of the site (backend and frontend) works fine.

I already have deactivated all plugins we are using (there are only a few) but this did not solve the problem. Therefore my idea is that this issue stands in connection with the multisite-installation. Did you expierience anything like that before?