Can only link pure text, no additional characters

So… I have used this plugin for a while, it worked perfectly, I used it to link books (set the titles in “” as text) and authors (two links for each, one with author’s name with a space at the end as text, the other with author’s name with an s at the end, otherwise genitives would not link right).

Then I added a book that had a / in the title and it broke the plugin. Couldn’t figure out what exactly broke the plugin so I uninstalled and reinstalled and added the links again and now links won’t work anymore! Or, they do, but only it the text contains nothing but plain letters, no spaces at the end, no “, no ä, ö, ü or ß (which is kind of impractical since the blog is in German and there are a lot of umlauts in German…)

Any idea what might have caused the links to stop working?

https://wordpress.org/plugins/wpmu-automatic-links/