Array to string conversion error when editing category

[Retrific]

With a new install of 1.6.1.0, the following error is displayed when editing a category:

 

Notice on line 1473 in file C:\Inetpub\vhosts\... ...\httpdocs\classes\controller\AdminController.php
[8] Array to string conversion

 

Also, when creating a new category, the "Parent category" list is not populated.

 

Thanks.