I was having the same issue with a G5 running 10.4. Only it was only happening with a standard user and not the administrator account. I checked the Adobe site and this is what they said:
Error: "Error in...Line 1:...IO Error" when you start (Illustrator CS2, Bridge 1.0.1, 1.0.2 on Mac OS X)
When you try to start Adobe Illustrator, you receive the following error: "Error in. Line 1: IO Error."
You logged in as a standard user.
Solution: Change the permissions on the Adobe folder.
1. Log in as the root user.
2. Select the Adobe folder in Library/Application Support, and choose File > Get Info.
3. Expand Ownership And Permissions, and then expand Details.
4. Change the permissions for Owner, Group, and Others to Read & Write.
The only detail left out is that you must apply these settings to all items within the folder.
This worked for me and I hope it works for you.