Are you using a particular WebDAV client or just the modeshape-webdav webapp ?
I tried it in the modeshape-webdav webapp in Chrome in Windows. From the Chrome network log:
Status Code:404 Not Found
Request Headersview source
I also tried the WebDAV client in Total Commander in Windows. And in this case I also get a 404 status code (as above).
In both cases I can see the file listed in the repositry just fine as a+b+c.docx. The problem occurs when I try to download it.
Can you please log a JIRA and we'll investigate this at some point. I suspect it may be related to URL encoding.