Problem scaling objects

Lately I experience problems scaling objects in Illustrator.

I created an 8 × 8 mm rectangle and tried to center it on a 10 × 10 mm canvas. What Illustrator gave me was a 7,761  × 8,114 mm rectangle, that wouldn’t center (It’s slightly off). Whenever I try to scale the rectangle to 8 mm it snaps right back to 7,761  × 8,114 mm.

I activated and deactivated every align-to-grid function I could find, nothing helps.

What am I doing wrong?

I’m using Illustrator CC on a Mac with OSX Mavericks.

Cheers

Corvin