Multiserver Setup on Ubuntu 18.04 - Cant add second server

I installed the master server as the web server per the Perfect document so to speak (meaning I followed the Perfect document for Debian but used all the commands from the Ubuntu 18.04 document in the order that the Debian document wanted) and Manual 3.1.
Now Im installing the second server, the mail server, but when I run the ISPConfig install in expert mode and tell it to join an existing setup, I get the following errors:

PHP Fatal error: Uncaught Error: Call to undefined function mb_detect_encoding() in /tmp/ispconfig3-stable-3.1-3ce6d765579b3698efe89259a31dcc74635ca609/install/lib/mysql.lib.php:448
Stack trace:
#0 /tmp/ispconfig3-stable-3.1-3ce6d765579b3698efe89259a31dcc74635ca609/install/lib/mysql.lib.php(166): db->escape('validate_passwo...')
#1 /tmp/ispconfig3-stable-3.1-3ce6d765579b3698efe89259a31dcc74635ca609/install/lib/mysql.lib.php(231): db->_build_query_string('SELECT plugin_n...', Array)
#2 /tmp/ispconfig3-stable-3.1-3ce6d765579b3698efe89259a31dcc74635ca609/install/lib/mysql.lib.php(270): db->_query('SELECT plugin_n...', Array)
#3 /tmp/ispconfig3-stable-3.1-3ce6d765579b3698efe89259a31dcc74635ca609/install/lib/mysql.lib.php(319): db->query('SELECT plugin_n...', Array)
#4 /tmp/ispconfig3-stable-3.1-3ce6d765579b3698efe89259a31dcc74635ca609/install/lib/installer_base.lib.php(239): db->queryAllRecords('SELECT plugin_n...', Array)
#5 /tmp/ispconfig3-stable-3.1-3ce6d765579b3698efe89259a31dcc74635ca609/install/install.php(297): installer_b in /tmp/ispconfig3-stable-3.1-3ce6d765579b3698efe89259a31dcc74635ca609/install/lib/mysql.lib.php on line 448

All of the remote users were created on the master server (root@ip and [email protected]) like the instructions said.
Mariadb was installed instead of MySQL and PHP 7.2, so where ever the commands were to install the mysql-client and mysql-server they were replaced with with the mariadb stuff and all of the PHP5 was replaced with PHP 7.2.
Are these errors above due to "mysql" not installed since MariaDB was installed in its place?

 

Thanks Till, as that fixed it. However, where is it normally installed in the procedures as I went back and looked at everything and didnt see that listed anywhere to be installed??

 

Ok this is very very aggitating!!!
In the beginning, I couldnt get the second server to add, the issue was missing the "php7.2-mbstring" since it wasnt in the old document about how to build the 5 server setup. No big deal....added php7.2-mbstring and was able to continue on with adding all the servers and got the WHOLE 5 server setup completed. Added all the domains and mailboxes so that I could move the old data over from the previous all-in-one server.
But, then I decided to update the Roundcube in the new environment from 1.3.6 to the 1.4.2 since I have 1.4.1 in the old environment. Well, bad on me I did it wrong and it messed up the "Web" server to the point that I couldnt get Roundcube to even get to the login screen. Again, bad on me for not taking a snapshot before.
So now I built 5 fresh Ubuntu servers to rebuild again.
I built Server1 (Web server + ISPConfig Interface) the same way I built it before and even got Roundcube 1.4.2 on and everything works fine, just couldnt test logging into an actual mailbox since the email server wasnt up yet but could get to the login screen.
Then I built Server2 (Email Server). However, when I ran the ISPConfig install and got the part of adding it to the Master server I get:
========================
PHP Fatal error: Uncaught Error: Call to undefined function is_arary() in /tmp/ispconfig3-stable-3.1-a5cdc4751b17e972f47d98a496a9d6176c399148/install/lib/installer_base.lib.php:529
Stack trace:
#0 /tmp/ispconfig3-stable-3.1-a5cdc4751b17e972f47d98a496a9d6176c399148/install/lib/installer_base.lib.php(400): installer_base->grant_master_database_rights()
#1 /tmp/ispconfig3-stable-3.1-a5cdc4751b17e972f47d98a496a9d6176c399148/install/install.php(321): installer_base->add_database_server_record()
#2 {main}
thrown in /tmp/ispconfig3-stable-3.1-a5cdc4751b17e972f47d98a496a9d6176c399148/install/lib/installer_base.lib.php on line 529
=========================
Now keep in mind, php7.2-mbstring was installed.
So how in the hell do I get these errors about permissions when all the permissions were created for servers 2 thru 5 as the following:
'root'@'ip2' and 'root'@'server2'
'root'@'ip3' and 'root'@'server3'
'root'@'ip4' and 'root'@'server4'
'root'@'ip5' and 'root'@'server5'
This is the same as I did before. So Im getting frustrated in why I cant get the 5 server setup installed again when I did it just the other day before Roundcube mucked up my server.
Can someone please shed some light as to why there is a permissions issue when the permissions were set???
So now I've put Server1 and Server2 back to the original Ubuntu build snapshot so I wouldnt have to keep rebuilding the original build. So now all 5 servers are at the default Ubuntu build and ready to start again, but I need someone to shed some light as to what is going wrong before I attempt starting again. Because when you're "this close" to being done and one thing mucks it up and you have to start over again and following the same procedures fail, it gets a little nerve racking.

 

Till,
I just noticed, the current version of ISPConfig is 3.1.15sp2, however, I've noticed the last couple of times I've downloaded the software from the link in the Perfect document for Ubuntu 18.04 the version is 3.1dev. Should the Perfect document be pointing to a "DEV" version??

 

Ok. This time I chose to go ahead and use the official 3.1.15p2 package and got all 5 of the servers up and running without any problems this time, in addition to getting Roundcube 1.4.2 + ISPConfig3 addons + Billing module 2.02 installed. Just have to migrate things now.

But is there a estimated time that either 3.1.15p3 or 3.1.16 is coming out with the bug fixes that you guys have been working on?

Thanks.
Rob.