Just reset your password for user b. even though you think you know the right password, if the preboot tells you the password is incorrect, then it must be.
It could be keyboard layout problems etc, but if the preboot rejects the password, the what you are typing is not what it needs to decrypt the key.
Now the preboot gives me 30 min timeout when I write in password. The keyboard is OK because I checked typing of letters during writing the username. I used a password for userB which was synchronised during LDAP synchronised task (password from AD not default EEPC password which is 12345)
I think you misunderstand the product - it's not possible to sync passwords from ad to eepc, ad does not know what your password is. Password sync happens on the client itself when you enter them.
Your best bet is to simply go through a password reset for this user, or try 12345 if you've never set a password for this account.
Remember, if you used user b before, the password will still be set. Eepc does not keep a separate password for every user/machine - passwords for users trend to the same wherever that user is assigned.