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.