It's often recommended (here in the forums) to be using the latest versions of Joomla/Community Builder. It's also recommended to disable any 3rd party extensions (one-by-one) to see if that's the cause of the problem. Also, making sure you use the default Joomla and Community Builder templates during troubleshooting will often help in finding the cause of the problem
The calendar uses Javascript - which means that it is possible that there is a conflict with another script that is being loaded. If that's the case, following the above steps might help in finding the source of the error.
Good luck