I too am having this same problem. I can say however, the problem is in the categories.
If you changed your homepage in the menu to a Portfolio category, other than the main Portfolio page, you will get this error.
I created a new category and assigned an old article, and still had the same problem.
The Template I am using is tz_portfolio_plus_demo - Default,
My Template version is 1.0
I Tried updating to 1.1, bu got the following:
Warning
Unable to detect manifest file.
Error
Error installing COM_INSTALLER_TYPE_TYPE_TZ_PORTFOLIO_PLUS-TEMPLATE