Failed to remove SSL domain(s): Sorry, nobody is an invalid domain

IF you are getting this error while removing ssl host through WHM then you need to check in httpd.conf if such user with "nobody" is there. make backup and httpd.conf and then remove virtualhost entry for that incorrect user and restart apache.

The error should be vanished now.

Horde error : 500 No response from subprocess with exit signal: 0

You need to check if the server is having custom mysql version? like below?

root@server2 [~]# mysql --version
mysql Ver 14.16 Distrib 5.2.12-MariaDB, for unknown-linux-gnu (x86_64) using readline 5.1
This custom version of MySQL conflicts with the internal cPanel PHP, so I disabled the "usecpphp" file so that Horde could send email properly:
root@server2 [~]# 

in that case you need to do

root@server2 [~]# mv /var/cpanel/usecpphp /var/cpanel/usecpphp-disabled 

phpmyadmin error "Unknown language: en-iso-8859-1

You need to check if the account having custom theme like RVskin? if yes then change the theme to default cpanel theme and the issue should be resolved.

For further support you need to contact RVskin support as cPanel wont support third party software issues.

can login whm with root2 user

Have you added root2 as a reseller account? Be default, only root and resellers that have been provided WHM access have access to WHM. Adding a root2 user does not allow that user WHM access, even if you set that users UID to that of root, i.e. 0.

In short its not possible to access WHM with root2 user.

munin 404 error

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HTTP error 404

The requested page was not found.
Possible reasons why you are seeing this page:

A bookmarked URL may have changed since you last visited.
The URL was entered incorrectly.
The URL was entered with inaccurate capitalization. (URLs are case sensitive.)

Please re-check the URL you are trying to reach. (Go Back)

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Resolution :

You need to make below changes at : /etc/munin/munin-node.conf 
from:
host *
To:
host 0.0.0.0