[Typo3] cannot log on admin after upgrading

[Bing Du]

After upgrading from typo3_src-3.8.0beta2 to typo3_src-3.8.0, I'm not 
able to log on as admin anymore.  Analyzing database in the install tool 
shows the new typo3 version recognizes the database fine.  Also, if I do 
'create admin user' in the admin tool, a uniqueness error returns.  So 
it knows the user admin already exists.  Changing back to the old beta2 
version, admin works again.

What's wrong?  Anybody have seen such a problem?

Thanks in advance,

Bing