You may see an error like the following in the roundcube log while attempting to access webmail via Roundcube.
[21-Oct-2020 23:41:56 -0500]: <eb64e94e> IMAP Error: Login failed for firstname.lastname@example.org against localhost from 188.8.131.52. AUTHENTICATE PLAIN: Account is temporarily unavailable. in /usr/local/cpanel/base/3rdparty/roundcube/program/lib/Roundcube/rcube_imap.php on line 200 (GET /cpsess5804459529/3rdparty/roundcube/index.php)
This can occur if the cPanel account has been transferred to another server using the Live Transfer feature, but an error is occurring on the server the cPanel account was transferred to.
If the cPanel account should be on the other server, the issue will need to be investigated further on that server.
If the cPanel account should be on the server showing this error, you will need to remove the proxy backend for the cPanel account. This can be done with the following command, where USER is replaced with the cPanel account username.
whmapi1 unset_all_service_proxy_backends username=USER
Additionally, if the mail should be hosted on the server as well, you should remove the MX redirects per the following article.