Here's a weird one. I use a network shared calendar called Remember the Milk. I pasted the URL into the options, gave it my username and password, and then I close it. I get a message saying
Error downloading the data from the url: webcal://www.rememberthemilk.com/icalendar/teensy/
Server returned result: () unknown error
I open the options again, and take a look, and my password is wrong. I can't read it, of course, but it's many characters too long. I change it and the same thing happens.