Try reinstalling K2 and make sure you are using the latest version of Joomla! (currently 3.1.5)

thanks - but I've done that already in a previous message. I just found a solution which was to install the legacy.zip patch - it solved the issue but I don't understand what legacy is nor why it worked.

The legacy patch is for the previous version of Joomla! (3.1.4) which had bugs. 3.1.5 fixed these issues

Strange I did have 3.1.5 and latest k2

I would be interested to see if anyone else has the same issue and can identify similar components.

Because of a bug in JViewLegacy::getName() all components with views using camel
cased class name break with error:

500 Layout default not found

and this is the thread
joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=31565

I cannot think of a reason however why your version did not have the fix for this bug.