Hey there! This is a known issue that's being addressed here:
Updates will get posted to the main thread at
In Progress - EA-10751 - cPanel check mail isn't working if logged in to cpanel.domain.tld or you can follow that article for updates.
Yeah, I saw that thread but noticed he was having the error on the "paper lantern" theme instead of the "jupiter" theme.
Also, his fatal error was different than mine:
[INFO]=[Banner: plugin not found]
vs
[INFO]=[Over standard port expect “cpanel.” service subdomain, not “hostname.domain.tld”! at /usr/local/cpanel/Cpanel/WebmailRedirect.pm line 260.]
Lastly, he only got the error when logging in cpanel.domain.ext; I get the error no matter what URL I login into (e.g. webmail.domain.ext, cpanel.domain.ext:2096, domain.ext:2096, hostname.domain.ext:2096).
The only work around I have is actually switching back to "paper lantern" theme and webmail will work as intended.
So while the errors are probably related, just thought there was enough differences to warrant a new thread or at least bring to light the warning error I am getting as well.
Thank you.