I remember reporting this one wayyyy back in the L4 beta group. It occurs randomly when there is an existing Logos Icon pinned to the task bar.
At that time there was less information on the dialog box.
Today it comes up with:

The information presented is more robust, but still sounds like programmer speak.
I'm assuming that means only that the label could not be changed from L4 to L5.
HOWEVER:
When I click it to load two things happen simultaneously.
1. L5 begins to load
2. I get a dialog saying "Can't open this item... want to remove it?
I expected the first, but not the second. For efficiency's sake I am going to remove the L4 icon and set a new L5 one.