Admin-ajax 500 in frontend submission (Premium)

I just started to set up your frontend submission form in a multisite and therefore activated the following fields:

• event_date
• event_end
• event_title
• description
• submitter
• event_allday
• categories

When I click submit as an admin nothing happens and the browser console shows: */wp-admin/admin-ajax.php 500.

Furthermore, the Date Picker outputs a different date format (YYYY/M/DD) than the input field specifies (YYYY-MM-DD). Even changing the date format manually does not change the problem. But I would like to have a European date format anyway (j. F Y), as defined in the text settings.

Then a few additional questions:

1. How can I display users a calendar just with their own submitted events in the frontend? The shortcode documentation doesn’t explain that.
2. Would it be possible to remove the time or at least use a placeholder for that?
3. Can I add custom fields to the submission and detail forms? I would like to use the plugin for an absence calendar in the team. Therefore, I would like to automatically calculate in a field the days how long a user is absent according to the form submission and then sum them up in a separate admin overview, which I want to develop independently.