Webmail Login Attempt Failed! on addon domain email accounts

[nishkrant]

1. Webmail Login Attempt Failed! on addon domain email accounts, but working fine on other domains and main domain.
2. This happened after transfer from 1 server to another
3. We have root access to this server
4. fixpop fixes issue but it changes password, and i am not sure if it can be done for existing email accounts as mails might get lost
5. If fixpop functionality can be applied to existing email accounts without password change, it will resolve issue

[cPanelDon]

1. Webmail Login Attempt Failed! on addon domain email accounts, but working fine on other domains and main domain.
2. This happened after transfer from 1 server to another
3. We have root access to this server
4. fixpop fixes issue but it changes password, and i am not sure if it can be done for existing email accounts as mails might get lost
5. If fixpop functionality can be applied to existing email accounts without password change, it will resolve issue

What is the full cPanel version number of the source and destination servers for the transferred account?

Have the symptoms been experienced on any other recently-transferred cPanel accounts?