Add New PHP Code Snippet not working

I am working on a multisite. The plugin is working fine on all 4 initial sites for 2 years now.
Now I have to add another site (language) and there the plugin fails to add PHP Code Snippets.
After creating a new PHP Code Snippet, the green bar appears with the message: “PHP Snippet successfully added” but nothing shows up in the listing of the snippets. Even with the most simple calls like ‘get_the_date();’.
I’m basicaly copying the working snippets from one of the other sites in the multisite network, so an error in the creation of the snippets should not be the problem.
Also the search for a tracking name returns no results.