Erik

Actually, there is a error message while submitting the contact. As signed in user I get this one:

FEHLER: Der Kommentar konnte nicht gespeichert werden. Bitte versuche es sp?ter nochmal.
? Zurück

ERROR: The comment couln't get saved. Pease try it again later.

The error message for anonymous user seems to be more significant:

Du schreibst die Kommentare zu schnell. Bitte etwas langsamer.

You are writing the comments to fast. Please slower.
That sounds like a anti spam plugin but I don’t really have a plugin doing that.

Wordpress Version: 4.9.8
Plugin-List:

• Contact Form 7
• Email protector
• Enable Media Replace
• Gutenberg
• Imsanity
• Responsive Lightbox & Gallery
• CF7 Invisible reCAPTCHA
• Widget Visibility Without Jetpack
• WP-Piwik
• Yoast SEO

adding new users works totally fine, the issue only affecting the comment table of the subsite.

Do you know, where does wordpress takes the running integer for comment_id from?
The source code (comment.php, comment-template.php) didn’t make so much sense for me…

Hey, thanks for the quick help.

If plugins are fine then try switching to the default theme and then check.

It’s not the theme (Publication, tried with default Twenty Seventeen).

This can be due to any plugin conflict, try disabling the plugins one by one and commenting to check which one created the issue.

I’m still on it but it’s no plugin only running on this subsite. I’ve forgot to mention, it’s a site in a network and it’s working fine on the other sites.

It’s also possible that your user table might have broke. It’s supposed to start with one and increment everytime a new user is added.

If you are comfortable looking around at the database with PHPMyAdmin then you can probably see the user table problems and a lot more comments that you can see from USER ID 0.

The both user there are fine (ID 1 and 2) but I didn’t added some for ages.
But it’s not the user ID making problems, it’s the “comment_id”.

I’d try adding a user and looking for that user. If they are not visible after being added you have a problem. Look at any plugins that work with the user table.