The crux of the issue is that PHP still can't write to that silverstripe-cache directory, even though you've assigned permissions. I haven't seen this problem on dedicated Windows server setups, but I'm not entirely familiar with shared Windows servers, as things may have been modified in the web server setup to accomodate multiple users.
I think it's time to ask your host why PHP cannot write into that silverstripe-cache directory even though you've set the permissions correctly. You could just forward them the errors PHP is generating with "Permission denied".
Sean