Note that the layout error only appears if there is a scrollbar in the grid view, or so it seems. If you delete a couple of data rows, so that all the remaining fit in the view, layout works without problems.
Also 'height' instead of 'minHeight' is a possible workaround, but it certainly doesn't have the same semantics and I need *minimum* height (it depends on size of another component).
Here is what I see, Google Chrome 22.0. The header is 50 px, but it is empty (no text, no background), and there is "Layout run failed" error in the console. Depending on font size you may need to add a couple of lines in the grid so that scrollbar appears.
I tested it with 4.1.2a (latest released version, as I understand) and also the nightly build from 22.10.2012. In both cases I can reproduce my error. But, cannot reproduce if I just run the code on e.g. Ext example pages on Sencha website...
So, can you please test with this HTML page? It is self-contained except you may need to alter the two Ext CSS and JS paths.