Setup Failure Codes/PHP curl support
The page identifies a minor configuration problem. Some functionality may be impacted but most parts of DAViCal should work fine.
The PHP curl module is required for external binding to work properly.
Unless you install this you will not be able to add externally bound resources or update any pre-existing if this module is missing.
No other functionality is affected if this module is missing.
Debian & derivatives
On Debian and derivative distributions this module is called "php5-curl".
sudo apt-get install php5-curl