Well, that did not do it for me.  I have a very similar problem.  I think I caused it by copying all my database files from my old server to my new one when I upgraded.
Here is the message I get when I try to access webmail.  It is similar, but with a different error code:
A fatal error has occurred 
object(db_error)(7) {
  ["error_message_prefix"]=>
  string(0) ""
  ["mode"]=>
  int(1)
  ["level"]=>
  int(1024)
  ["code"]=>
  int(-24)
  ["message"]=>
  string(24) "DB Error: connect failed"
  ["userinfo"]=>
  string(86) " [nativecode=Access denied for user: 'horde@localhost' (Using password: YES)] ** Array"
  ["callback"]=>
  NULL
}
[/home/httpd/html/horde/lib/Prefs/sql.php : 102] 
And resetting the root password for mysql doesn't solve my problem.