Heiner,
Thanks for getting in touch. I was able to successfully submit an event without an end date in my test environment, since the field is not required. Let me know if you’re able to do this on your site.
Regarding your second question, yes you can edit how the date format displays on the front-end by doing the following:
1) Make sure you do a template override (https://theeventscalendar.com/faq/what-are-template-overrides-and-how-do-i-do-them/) before making any modifications
2) In your template files, look for some PHP that looks like this: https://gist.github.com/3879151 and change it to this: https://gist.github.com/3879165
Let me know if that does the trick or if you have any further questiosn. Thanks!