Submit Draft problem

I had very intriguing situation last week, while developing PoC application using PPS2007. We set up everything (including models, forms, assignments etc.) and tried to enter some data using Excel add-in. Everything went fine, until one of the users selected "Submit Draft" action from the Actions drop-down. When he submitted data, he couldn't get "Submit Final" option anymore!
After searching a while, we have found several solutions:

1. Stop Planning Service and IIS and and do an ALTER application database enabling broker.
2. Configure Service Broker.
3. Set up Planning Service to use another domain account.

All with no success. Same time, there was a bunch of errors in Event Log, fired by SQL Server. Errors go something like this:

"An exception occurred while enqueueing a message in the target queue. Error: 15404, State: 19. Could not obtain information about Windows NT group/user 'domain\user_name', error code 0x5."

where domain\user_name is a domain account under which SQL Server is working. Actually, this problem is not an issue of PPS but of SQL Server. We set up SQL Server to run under Network Service account. If any SQL Server entity needs to access Active Directory domain controller (e.g. from stored procedure or via Service Broker like PPS does), SQL Server must be run under domain account! So we changed SQL Server to run under domain account and everything went fine.
This is was very interesting for me, since I've tried to follow all recommendations from Deployment Guide and this topic wasn't mentioned.