Lightroom behave very strange to me. What you say (click and drag) works approximately every third or fourth time I try. Holding space doesn't work since that toggles zoom in/out.
This happens both in Library and Develop mode. Can you swith to the hand tool manually?
To my observation there seem to be a few different cases:
It seems fairly predictable to me.
Thank for a quick answere Simon. I guess it's 'just' a bug. The only thing I had to do was to restart Lightroom. I had some pictures importing before and that's why I couldn't try that option before..