'recent posts' widget doesn't work! Help!

https://www.bewustzijnsvormen.nl

I put the recent posts widget in the 1st widget space. It creates a link like it’s supposed to (see my webside at the right sidebar, underneath Agenda the link says ‘test a’). But this link doesn’t work! It goes to the error page! (to the URL: https://www.bewustzijnsvormen.nl/http:/www.bewustzijnsvormen.nl/pagetitle instead of https://www.bewustzijnsvormen.nl/agenda/test a or something.)

I’ve a static homepage and my posts are visible on the page ‘Agenda’.

how can I fix the widget, so it starts making correct links?

Thanks so much for your help!