PHP Issues While Upgrading To 4.1.15

[ModSquadHockey]

Our forum:

• IPB 4.1.14.3 
• On a VPS running CentOS 5.9
• Plesk 12.5.35
• PHP 5.5.38

We're wanting to install the 4.1.15 upgrade. The installer finally gave us the news we'd been dreading: that PHP 5.6 would be required, which wasn't available in our version of Plesk. We contacted our VPS provider and they manually added PHP 5.6.26 and registered it with Plesk so we could use it. 

The issue we're seeing though is that as soon as we change the existing forum (again, on 4.1.14.3) our forum craps out. Just spits out:

Fatal error: Call to a member function execute() on null in /var/www/vhosts/modsquadhockey.com/httpdocs/forums/system/Db/Db.php on line 411

What am I missing, here? 

Obviously the long term goal here is to upgrade the VPS with a newer OS, etc. We're already in communication with them on a migration plan. But in the mean time, we really want to get the forum patched for security reasons. 

[TSP]

Where does it spit that error out exactly? Everywhere? Does it show only on the upgrade page? Only the frontpage? Does it show on a topic page? If you have a link to a topic page you know exist. Are you able to access your admin CP? 

Is there no backtrace of the error saved in uploads/logs/ or in the database table core_logs?

[ModSquadHockey]

On 10/5/2016 at 5:52 PM, TSP said:

Where does it spit that error out exactly? Everywhere? Does it show only on the upgrade page? Only the frontpage? Does it show on a topic page? If you have a link to a topic page you know exist. Are you able to access your admin CP? 

Is there no backtrace of the error saved in uploads/logs/ or in the database table core_logs?

Sorry for the delayed response. 

It spits out the error regardless of URL. Main forum page, topics page, admin CP... everything.

Nothing in uploads/logs/. Sorting by date on that directory, the last file modification was yesterday. Nothing gets logged in either the Error Logs or System Logs page of the admin CP.

I don't know where else to look at this point.