Well , after a few hours of trying the problem is solved. I didn't set up correctly in Excel.
I had a list of two columns of names which I want to combine with 'UndirectedEdges' In Excel I already concatened the strings and imported them in Mathematica. But what I overlooked is that Mathematica sees the concatenation as one string and not as two separate string.
So I splitted the names over two colums and now the import works as expected.
Thank you for your time,