Tumblr to WordPress: Permalinks go wrong

Hi All,

I recently moved my tumblr blog to a clean wordpress installation with the tumblr to wordpress this export tool: https://haochen.me/tumblr/
Everything went fine, except my permalinks, they are still messed up. The only way to change them is go in every post, change the permalink manually and save the blogpost.
I already went to settings > permalinks and changed it from numbers to: date > post title. But the wont update.

There are over a 100 posts at my blog so doing them all manually is just not possible. Is there a way to solve this?

This is the url to my blog: https://www.jonassmets.com the first 2 posts are changed manually, so those permalinks are fine. The rest is still messed up.

Hope to find a solution here.
Greetings
Jonas Smets