login failed

Hello!
i have MT Router 2.9.6 before i enabled the web proxy and everything is working well without any errors.
but after enabling the web proxy some of users cannot authorized the hotspot and error messege in the LOG said:
(( Login Failed: Password is not chap encrypted ))
Please any advice and thanks in recent

First of all upgrade your router to 2.9.40 version,
then provide us with exact information from the logs.