I can log into the web UI OK, but I can't seem to access calendars. What is going wrong?
The Web UI is a browser-based system using normal GET and POST requests and a cookie-based session scheme.
The actual CaLDAV interface uses many more different HTTP methods (PUT, PROPFIND and REPORT in particular) and it uses HTTP "Basic Auth" for authentication. There are several things that could be going wrong.
One of the commonest reasons for this is that the HTTP headers are not being fully passed through from your webserver into DAViCal's PHP code. This can happen if you're using an interface like FastCGI to execute DAViCal's code rather than the PHP module for Apache.