I think this is a bug that appears on some systems, from the web-client code that is using the wrong transaction setting. Does the message appear when you first login? If so then it's the problem. It's been fixed in the SVN codebase. To fix it before the next release, you can download the Alfresco Source, build and patch it on your machine:
In the class:
org.alfresco.web.bean.UserShortcutsBean
and the method:
public List<Node> getShortcuts()
change this line:
tx = Repository.getUserTransaction(context, true);
to this:
tx = Repository.getUserTransaction(context);
Or if you are happy using Subversion to update your source, then update to SVN revision 2074.
Sorry about that,
Kevin