Changing the permalink settings causes error

Hello to all,
I’m new in this forum and would need the help of you.

First the basics:
I use WP 3.9.2 with the theme Yasmin. There is a slider in the top of the page.

Unfortunately, I have only now found that in the basic setting in WP Permalinks are set to “Standard”. Now I changed this to post name, but unfortunately this does not work with all side links.
Here’s a link to my page. I think this makes it more understandable.
http://balticnet-plasmatec.netzwerkstelle.de/
If I try to open the “Projects” in the slider, “Projects” at “Latest News” or in menu “About us”, the link does not work.
“Projects” and “Working groups” are not working, all the other sliders and menus are working (e.g. “Events”).
Unfortunately, I can notfind my mistake, please can anybody help me with this?
Just one more very short question, is there a plugin that shows me which of my pages do not work (“Not Found!”)?
Thank you so much in advance.

Katherina