Don't worry, this is not an error, but a notice.

You just have to turn off or set on default your error reporting in joomla configuration (as it should always when a site online!)

Due to its cross-plateforms 2.5 and 3.1 feature, some notice could appear in one joomla, and not other.
Some could be hidden in iCagenda code, but other not.
This one you report here can be hidden, and will have a look to hide it (but don't worry, iCagenda is working without problem for this feature )