Previous vervsions were or are installed?
If you start in Safe Mode (SHIFT key down) it does the same thing?
If you create a new user, login as that user, it does the same thing? (points to a System rather than User Account problem)
If you create a New DB with a single table and a single field, it does the same thing? (eliminates corrupt database as cause)
If you disconnect from the network, it does the same thing?
Not Relevant to your problem, but an example of symptoms caused by an OS X problem.
FileMaker Pro 8.5, FileMaker Pro 8.5 Advanced, Mac OS X
On the Mac, the FileMaker Web Viewer is driven by the Safari Web Browser and Apple's built in Web Toolkit. Because of this, FileMaker has needs that extend beyond the FileMaker application. This problem usually extends from a corrupted file within Mac OS X called 'com.apple.internetconfig.plist'. Here are the directions on how to remove this file and restore Web Viewer Functionality to FileMaker:
Quit Safari and FileMaker Pro and/or Advanced
Go to /Users/your_user_name/Library/Preferences/ and drag (don't empty yet) the 'com.apple.internetconfig.plist' file to the Trash.
Relaunch FileMaker Pro and/or Advanced and open any database with a Web Viewer Object. Go to the layout with the Web Viewer Object and view it in Browse Mode.
If the layout opens without crashing you can empty the 'com.apple.internetconfig.plist' file from the trash.
Note: This issue has been resolved in FileMaker Pro 9 and FileMaker Pro 9 Advanced.
Thanks David. Before I go through these tests - I do have multiple previous versions (and the same problem occurs when trying to turn on IWP in each). Do I need to remove previous versions of FM?
It is likely to be common shared plist, object (font), or process (networking).
Moving a copy to another computer, testing and maybe moving to another network and testing might help diagnose.
Safe Mode startup takes 5 minutes. The create new user takes 10 minutes. I would start there.