Thank you for sending a screenshot to info@soscisurvey.de
It looks like you're trying to send the emails "immediately", but JavaScript does not actually start. There are different solutions for that:
(a) make sure that you have not disabled JavaScript in your browser.
(b) reload the page using F5 ("reload page")
(c) clear the browser cache, something in the cache blocks JavaScript in a certain page to run correctly.
(d) try another browser - it should work in any browser, but if one is broken...
Another solution would be not to use the "send immediately" option, but to schedule the mailing. You can use a time in very near future (e.g. in 2 minutes).