Thank you all for the reply's. I did the upgrade to 5.1.94 with the file1 and file2 links.
And after upgraded without problems to 5.4.27. I just found the instructions a bit confusing, if it would say ….
OR if you are updating from firmware 5.1.8 or older, download both files below (.tar and .tar.sig) onto the root folder of your USB stick
Thanks.
Ruth