The linked forum provided the solution (by sircali): Unable to log into Duolingo App - #17 by sircali
The trick is to ask for password recovery in the application. You then open the email using (in my case) Firefox browser and long-click the recovery button in the e-mail until context menu appears where you select open in external application. Use Duolingo where you will be prompted for the new password. Since then, the app is working! HALELUJA!
Thanks!