A few customers reported when opening an MS Word document on a WebDAV drive, Word pops up an error dialog: "This document could not be registered. It will not be possible to create links from other documents to this document..." Usually the document will still open without any problem. You can copy the file to a local folder, or copy it to another cloud folder. So what caused the problem? 

In trouble shooting the problem, we copied the file from the cloud drive to C:\, and we were able to open the file locally. We copied the file to a test folder "\test" in the same cloud drive, and we were able to open the file as well.

The problem is actually caused by the length of the file path. On Windows, the max file/folder path length (The UNC path length) is 256 characters. Due to UNC prefixes, the actual max length is about 248. This seems to be true for most file types. However, MS Office files are different. e.g. MS Excel limits the maximum UNC path length to 218 characters:

http://answers.microsoft.com/en-us/office/forum/office_2007-office_install/unable-to-open-excelxls-files-with-very-long/290c6e39-929a-4788-95c4-d3364a7060f7

Also MS office documents can embed other documents, so they need to reserve certain length for the extra links / references. 

You might notice the problem is more likely to happen with files on a WebDAV drive. This is because the WebDAV prefix is usually longer than 8 characters, so the maximum path length is reduced to about 220 characters for regular file types. For MS office files, it is even shorter at about 190 characters.

A very important factor is using special characters and SPACEs in folder names and file names. WebDAV uses HTTP/HTTPS; when MS Office opens a document file on a cloud drive, the request is sent in URL format. Any special characters such as "&, %, #, +, ?, *,',", >, <, /, \, |, :, SPACE" must be URL encoded. In doing so, the URL will be significantly longer. So we don't recommend using file names like "Let's have a meeting tomorrow.docx", because it contains the quotation mark. (though it actually works with our service!). We recommend using "LetsHaveaMeetingTomorrow.docs".

So the solution is very easy: just rename your folders / files to make the total path length shorter; avoid using any special characters and SPACES in file/folder names.