From Piwik 1.9.x to the current 2.0.3-B3, Piwik somewhen failed to render referrers (“external websites”) correctly. I’m stuck to this loading screen (see attachment) on all of my machines.
Any clues?
try to delet files from temp folder?
try to increase php.ini memory higer?
try to delete widget and re add it after above steps.
what php version are you using?
Temp folder cleared: No changes.
Memory limit already maximized, much more than I ever needed.
Readding the widget removes the “Loading…” but shows “no data” now. 
edit: Ah, that did the trick indeed. Thanks!
PHP 5.4.16.