Hello,
Sorry not to speak German!
About the nofollow, there's a good reason : to not save in search engine not real url!
Such url with nofollow are "action" url (changing month, add to cal, registration form...) which are url not to be followed by search engine robots.
This is a way to improve your own SEO, and a better page rank with no fake backlink
About the w3c validator, there were some issues with div.
You can now test it with 3.5.1, the latest version, as the reported validation issues were fixed
note that w3c validator should be run on HTML5 as iCagenda is HTML5 and responsive (so if your site template is not HTML5, the w3c validator setting for doctype could be wrong.)
Regards,
Cyril