You are sending text unencoded. urlencode that just like you do the permalink.
<a class="popup" href="http://twitter.com/share?url=<?php echo urlencode(get_permalink($post->ID)); ?>&text=<?php echo urlencode(the_content_limit(100, ""));?>"><img src="http://zitatezumnachdenken.com/wp-content/uploads/2013/04/twittersmall.png" alt="twitter"></a>
Although, the_content_limit looks like it probably echos (based on your usage) instead of returning a string, which you will need. So I expect you will have to find that function and alter it or duplicate it or find some other alternative to get a string that you can encode.