I managed to get OneNote notebooks from a OwnCloud/WebDAV drive by deactivating the file lock option. Now I get all information (pages, sections etc.).
But when I change one page in Outline on my iPad and try to sync these changes the corresponding page file in mey WebDAV folder is emptied (file size = 0 kb).
No error occurs, everything seems to work correct; except the fact, that the updated file is crushed/emptied.