We have applied a patch which we think addresses this problem
We think recent versions of Safari (and probably Chrome) on a Mac have a bug that is preventing files from being uploaded. This is affecting the Assignments tool (and Resources, etc.) and is stopping people from uploading essays.
Until we have had a chance to try out a fix, it would be very sensible to recommend that Mac users try to use Firefox with WebLearn.
For the more technically minded: https://jira.sakaiproject.org/browse/SAK-17255