Dropbox sync delete files

I use ReadyNAS 312 (fw 6.7.4) and have setup a network share to sync to Dropbox and it has been working great until now.
Now files are sporadically disappearing from our share (300GB, approx 230.000 files).

I can reproduce (almost always) the problem by:
1. Open a file e.g. Word.
2. Save as new file and close Word.
3. Looking in windows exlorer I can see file is disappearing from share in front of my eyes.
4. Looking in www.dropbox.com/events I can see 5 events regarding this:
a. Word file was added
b. Word file was added again,
c. Temp file was added
d. Word file was renamed to a temp file
e. A thired temp file was created.

We don't have this behavior on another share (same NAS device, not synced to Dropbox), so I suspected Dropbox sync to be the problem and when turning sync off the problem went away.

What could be the cause? Is it a bug in fw?

Thanks
Tommy