I have run into this problem even when I have not changed the password. I tried the fix to add the service account to the backup operators group and that did not completely fix the problem. Additionally, I have notice that I have had a lot of other mysterious SharePoint Server Errors What you need to do to correct this problem is:
1. Go to Central Administration > Operations > Service Accounts
2. Click on the Web Application Pool
3. Under Web Service choose Windows SharePoint Service Web Application.
4. Under Application Pool choose your Shared Services Provider application pool account. This should populate your application pool account under configurable.
5. Re-enter your password and click OK. You need to perform an IIS reset for this to take effect.