Hey Markus,
Both of the links you shared let me to “404 – Not Found” error pages on your site, so I wasn’t able to check out the pages directly. I’m curious though – do you mean that you downloaded a .po file for The Events Calendar from http://translations.theeventscalendar.com for your language, and activated it, and everything else translates fine except those two strings?
If so, what .po file are you using?
If the .po file itself is missing a translation, then the only solution is to add the translation and re-upload the modified .po file.
Let me know if you can make these pages live again – I will check them out. It should be helpful because it also possible that this un-translated title is being generated by your theme.
Thanks!
George