You need to relocate the NIUser-OneNote because the server has been moved to new hardware.
If you did not changed since autum last year, its maybe better to close the NIUser-OneNote and open the new NIUser2019-OneNote https://spf.gsi.de/sites/NIUser/Freigegebene%20Dokumente/NIUser2019/Notizbuch%20öffnen.onetoc2
If you are not sure, you can move your contributions to your Personal-OneNote, then close NIUser and open NIUser2019-OneNote. If any of your contributions are missing you can move them back from your Personal-OneNote to NIUser2019-OneNote.
-- HolgerBrand - 2019-02-01