We have several reports of Opera 7.23 and Macs (various OS versions) being unable to access calendars at our usual address, my.calendars.net/nameofcalendar. We do not know why this happens or whether it can be avoided by changing some setting in Opera or in the Mac broswer. But you can access the calendar with Opera 7.23 or a Mac by adding "www" to the beginning of the address, so that it reads:
http://www.my.calendars.net/nameofcalendar
If you figure out a setting within Opera 7.23 or a Mac that solves this problem, please let us know at support@calendars.net.