Hi

First I had my website in Dutch and English.

I want to change that to only English, but when I do so, I can't create a new page.
I get the warning: Page updated but you need to give it a title before publishing! And nothing is saved. Same with newsarticles.

When I active the Dutch language it's fine again.

So you had a multilingual site first and then switched to single lingual? We will try to reproduce that but please look at your error logs adn also look what is stored in the database.
Some info about that: http://www.zenphoto.org/news/multi-lingual-sites

I can indeed reproduce that if you have a two language multilingual site and then disable multilingual.

Thank you for checking. Glad it's not just me ;)
But what do I do now?

Sadly you have to wait until we found the bug and fixed it. But short fix is, don't switch from multilingual to single lingual mode.

haha lol, tnx!