[Plugin: ICS Calendar] Error opening ICS file

Hi,
I installed the plugin and tried to enter the URL of the ICS file in the Settings window, but no matter what I do I keep getting this error in red underneath the URL input box:

Error opening …(file path)…/wp-content/ics-importer-cache/805857d8b8531b468b654b002730a487.ics

Here’s what I’ve tried:
— checking the ICS file location (it opens in a browser as text)
— toggling permissions on /wp-content/, /ics-importer-cache/, and even the ICS file itself (747, and even 777 temporarily just to see if it would work, though I wouldn’t leave them at 777)
— putting the ICS file outside of the /wp-content/ directory, and even outside of the site itself, adjusting the URL in the settings box each time. (tried adjusting those folder/file permissions as well)
— deactivating the plugin and reactivating it

Does my .htaccess file need to be writable for this? I tried making that group-writable but still no dice.

Am I doing something obviously wrong here? (I have enough technical knowledge to be dangerous but not enough to be completely fluent in its intricacies.)

Thanks for your help!