Using “Less secure app access” is a big no no. the chances the sniffer intercept the data and decrypt the data is high. Experienced frequently notification from google that my google account was trying access from different country.
use the app password insert in the manager email setting. it will work.
The reason of error @Mark1 received because google only allows the access if the password is typed in their own website. Oauth login method. Manager doesn’t cater Oauth login method.
if you have the same experienced the same like being access from different country or different devices with using app password. you can dispose of it and regenerate new one. so your master password to access google account only exclusively used on google site.