What syntax do I have to enter for getting correct redirects? No matter if I enter for example /english/project-management or http://www.URL.com/english/project-management it won't work. Neither with source not with target URL. Trailing / or not doesn't make any difference either.
Or have I completely misunderstood this feature?
You have to enter a full URL http://..... or use * symbol to mark any number of any characters.
Be careful, it is possible to do an infinite loop or break admin.