I am sharing a dirty solution to the following problem:

I upgraded two different portals from 4.5.3 to 4.6 and in both cases the new portals did not recognize the passwords and did not let me log in neither as host nor as admin.

To solve the problem, I did the following:

succesfully created a new user

opened the online database with a database admin utility

went into the hostsettings table to make sure that the WhitespaceFilter is set to N

went into the aspnet_Membership table

copied the new user's values from Password and PasswordSalt INTO the values of the HOST administrator user

logged in with the host userid and the new user's password.

From there, you can force the change of passwords for all your users. The new generated passwords will be recognized by the new portal.

It sounds like when you did your upgrade that you might have missed the "machineKey" values when you were merging web.config settings.

You will want to be sure that those values are copied from one version to the next.