I created an article on the master site to link to our monthly send newsletters.
See: http://online-boten.nl/nl/oxalex/nieuwsbrieven

On this article I'd like to put more links to send newsletters to create an archive and link these to the new articles just made.

Click on this example
Augustus Nazomer.. Mooi weer om lekker te varen''
online-boten.nl/nl/nieuwsbrief-augustus-2011

when I go to the slave site "oxalex" I can see the changes to the article.
See: www.oxalex.nl/nl/oxalex/nieuwsbrieven

but when you click again on:
Augustus Nazomer.. Mooi weer om lekker te varen
online-boten.nl/nl/nieuwsbrief-augustus-2011

You see the link is not working 500 error which means article does not exist?

How can I fix it?
I tried everything and cant figure out, how to make it work.

None of the link that you provide result in 500 Errors.
So we suppose that you find the solution yourself.