While I understand what this error message means, I don't have enough information to determine why the error is occuring!
The error means that for some reason, the query to the database is not working correctly for some reason. So instead of being able to combine the data retrieved from the database, there is no data and the combine (merge) fails giving you the error.
This usually occurs when an unknown variable is added to the equation like a different authorization system, an incompatible component, or a server setting isn't compatible. The only way I can compensate for the error is by seeing what your installation of Joomla is doing at the time of the error.
Please read the following:
Instructions For Posting A Bug Report
Getting Debugging Information
Thank You,
vujsa