I am using 64bit 2.10.1-Pisa on Windows 10.
When I set up my composer to print, everything looks fine until I try to print or export. The composer goes completely blank and outputs a corrupted PDF using the Export to PDF function. The PDF file won't even open.
I had the same result exporting to PNG.
When I try to use the PDF document printer instead of exporting, QGIS stops responding indefinitely.
Best Answer
If you are using the OpenLayers plugin, don't!
It's got all sorts of issues with print composer and exporting maps.
Use "quick map services" plugin instead.