I would refer you to this site for a few tips on using rowfilter. http://www.csharp-examples.net/dataview-rowfilter/
In your case, I think the proper way may be to write the string like this. I have followed a similar approach and this worked.
...DefaultView.RowFilter = "Dubletten_Vorschlaege ='" & "'"
you will notice that i add a single quote after the = and again add a single quote within double quotes after the &.
Also, I refer you to this StackOverflow link How do I check for blank in DataView.RowFilter The user simply added the line of code as this:
...DefaultView.RowFilter = "Dubletten_Vorschlaege = ''"
notice the two single qoutes before the ending double quote.