I have the same problem. However, it is not related to keys, these were copied correctly. The problem with host and admin accounts appears to related to membership provider, in the old config the password length was set to "4", I failed to change this in the new 4.3 config which was set to "7", during the upgrade the I believe passwords were converted.
The password reminder does work for either account and any attempt to login throws errors.
However, a new user can register and login without any problems.
I believe the problems as listed above not only effects the host and admin accounts but all users in the database that had a password smaller than "7" characters.
I have also noticed since the upgrade that some pages will not display without errors to users unless they are logged in. The pages in this case could be viewed correctly before the upgrade and did not require a login. When a user that is not logged in visits the page they rec'd the "Exception has been thrown by the target of an invocation" error. However, the page is displayed correcectly if the user logs in.
I have not been able to correct this problem.