WebDAV iOS still return error with credentials

Version 1.0 (166)
If I verify the webdav server, it returns error that my credentials were refused by the server. They are certainly correct and works on the desktop client. WebDAV on Synology NAS.
If I go back to the library, icons of attachments shows white arrow down in blue circle. After download, it returns "unable to display content". I am not sure whether it is downloaded but the icon shows it is (arrow down is not visible, just the pdf icon).
Other pdf shows exclamation mark after the try to download it.
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