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
Date:                10/26/2007
Time:                10:26:31 AM
User:                 N/A
Computer:          MACHINENAME
Description:

Safe mode did not start successfully. Request for the permission of type 'Microsoft.SharePoint.Security.SharePointPermission, Microsoft.SharePoint.Security, Version=12.0.0.0, 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.