I ran into an issue where after deploying some custom solutions with a customer, the SSP site would not load. It would just display a message that said unexpected error. In the event logs we saw the following error.
Event Type: Error
Event Source: Windows SharePoint Services 3
Event Category: Runtime
Event ID: 6610
Time: 10:26:31 AM
Safe mode did not start successfully. Request for the permission of type 'Microsoft.SharePoint.Security.SharePointPermission, Microsoft.SharePoint.Security, Version=220.127.116.11, Culture=neutral, PublicKeyToken=71e9bce111e9429c' failed.
For more information, see Help and Support Center at http://go.microsoft.com/fwlink/events.asp.
The fix for this was to change the trust level in the web config from WSS_Minimal to WSS_Medium.