Hello,
I can login but not find a problem.
On this page:
http://schwyz.krt.ch/index.php?option=com_jevents
I click any of the + to create an event and I can.
The only issue I found was you had not closed the module you pointed to in your screenshot so a person was still checked in. When clicking the edit icon you were returned to the page and a notice was issues by Joomla! letting you know the person accessing the module is not the person checked into the module.
Checking in the modules on the backend allows it all to function. Again, this isn't a JEvents handling it's ALL Joomla!
And you may want to know about the feature:
Joomla! Admin -> Site -> Global Checkin
this allows you to clear all checkins. You are not checked out if you do not click 'Close' or Cancel or Save and Close in the editing of any article, module or plugin.
Many thanks
Tony