Have you configured Rainlendar to use the http or ftp server to access the events? The ftp server should be easier to set up since it doesn't require any additional components but unfortunately some ftp servers don't include all the functionality required by Rainlendar so they might not work properly.
Setting up a http server needs a bit more work. A standard http server does not allow updating of the files on the server so what you need to do is to install (or enable) the webdav extension to the server and configure it so that the folder where the Default.ics file is located has write permissions.
If you run Rainlendar in the debug mode the log
will contain all the communication between the client and server which can help when googling for the errors.