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.
Safe mode did not start successfully. Request for the permission of type 'Microsoft.SharePoint.Security.SharePointPermission, Microsoft.SharePoint.Security, Version=126.96.36.199, Culture=neutral, PublicKeyToken=71e9bce111e9429c' failed.