While testing stuff regarding Bug 112 "Synchronization Error - Couldn't delete local events" I've noticed that subscribing to calendars fails if the calendar file is a little bigger (~4MB). It keeps spinning "Validating calendar resource..." and never finishes.
When replacing the .ics file with a small dummy file (containing only one event), subscribing and then switching back to the original, lager .ics file works fine.
I suspect this issue maybe related to Bug 112 as it also seems to be dependent on the file size or number of entries?!