Activation lock problem
Since you removed that device from iCloud list of locked devices, just put the device in recovery mode and restore iOS from iTunes over USB. That wipes everything, including the cached old password. 


https://support.apple.com/en-us/HT201263


If that fails, post a follow up with the details of the failure like error message. 

You could also go through support if needed to prove if you are the original purchaser of the device. 


How can I bypass Activation Lock?