Hi Sebjo,
I must admit i'm not a very excperienced user of pictures in office docs, but i think i located the problem
Right click on the logo when you insterted it, choose format picture
On the picture tab, you should have a bottom below that says compress, click this, then again at bottom you have two options, compress pictures and crop.............;
turn both of these of, and save your file and reopen it
btw you will get a warning from word about saving disk space and bla bla bla, just ignore, you want this logo to look good
If this doesn't do the trick, i will need some time to find another solution
With regards
Mike