Not to worry, people. I figured it out myself.

The problem was that in OLAP2000 the default measure of the cube can be either a real measure or a calculated one.

In 2005 it cannot be a calculated measure, but the migration routine migrated the cube in the way that the default measure of the whole cube was one of the calculated ones.

I just clicked on the dropdown box there and it gave me only real measures to choose from, and that [Headcount] was not even in there.

ps: there is an article on the sequence of steps how a cube is built in MSAS2005. I should have read it more closely

here we go. this is it