This is a duplicate of https://forums.bitfire.at/post/12711. Please follow up there. Unfortunately, I don’t know why Android starts synchronization when Internet is not available yet.
I have a synced calendar for school using a system called Edumate. Attempting to log in with ICSDroid doesn’t redirect correctly, and produced a ‘302 Moved Temporarily’ toast. Visiting the ICS file in Chrome on my mac downloads it correctly.
Which ICSdroid version are you using?
Can you provide the URL?