I've played with this some more and finally gave up and reinstalled both Alfred and your workflow. No luck unfortunately. I get this same warning when trying to set my api key "Warning: Invalid argument supplied for foreach() in /Users/xxx/Library/Application Support/Alfred 2/Alfred.alfredpreferences/workflows/user.workflow.xxx/asana.php on line 438."
FWIW, I'm on OSX 10.9.2, 2.7 GHz Intel Core i7