"Nivo slider" is a good plugin for slideshow, but the version 1.9, upgraded today, comes with a bug - it shows a message:
Warning: array_key_exists() [function.array-key-exists]: The second argument should be either an array or an object in /homepages/19/d395780486/htdocs/wp-content/plugins/nivo-slider/nivo-slider.php on line 1572
So it needs some emergency surgery... Here is the quick fix: add a new row
if(!is_array($sources)) $sources = array($sources);
right before the row 1572.
Stupid... If it's stupid but it works, it ain't stupid! (Murphy's law)