WebCalendar removed the need for magic quotes on the version 1.1.1 release in 2006, this error is just from some of the installation pages that check for magic quotes and turn it off, then reset it.

you can edit the install/index.php file and comment out the lines as below.

//around line 113
//$magic = @get_magic_quotes_runtime ();
//@set_magic_quotes_runtime (0);

//around line 136
//@set_magic_quotes_runtime ($magic);

// around line 255
//$magic = @get_magic_quotes_runtime ();
//@set_magic_quotes_runtime (0);

//around line 274
//@set_magic_quotes_runtime ($magic);

edit the includes/classes/phpmailer/class.phpmailer.php for the same things at line 1090,1091 and 1095 You will also need to edit: install/install_functions.php at lines: 457,458 and 486