How can I get the iCal server to work correctly?!

We are a business that uses Apple software and products for everything! For a while we've been having an issue with iCal - it seems when an invite is sent we are unable to accept, we click 'accept' but the message is never sent, so then it looks like we're ignorning the request (which of course can look un professional). We have approached various IT companies and all they seemt o sugegst is using a different Mail Server (such as Kerio, Exchange etc) as a fix, which of course is quite costly, especially when we believe, that iCal should be able to work effectively for a growing business.

We've deleted users from the server, added them back on...the only way they can accept invites is if they keep it on the local calendar, which of course isn't any good to us as we share the calendars, therefore it needs to be on the server.

I'm really hoping there is something someone out there can tell me to help us! I am loosing hope and I am reluctant to fork out for such costly solutions as Kerio when I know iCal server can work. Please help! If you need any further information please let me know.

Thanks