Typo3 Extension ke_search unable to load configuration / templates

I'm trying to migrate a typo3 7.6.32 installation to 8.7.24. After updating all extensions to the most recent versions and having the basic page setup and working, the ke_search extensions breaks on one specific page only.

Actual Error Message:

 Argument 1 passed to TYPO3\CMS\Fluid\View\StandaloneView::setTemplateRootPaths() must be of the type array, null given, called in /var/www/web18/htdocs/relaunch2016_t3v8/typo3conf/ext/ke_search/pi1/class.tx_kesearch_pi1.php on line 95 

TypeError thrown in file
/var/www/web18/htdocs/typo3_src-8.7.24/typo3/sysext/fluid/Classes/View/StandaloneView.php in line 178.

16 TYPO3\CMS\Fluid\View\StandaloneView::setTemplateRootPaths(NULL)

/var/www/web18/htdocs/relaunch2016_t3v8/typo3conf/ext/ke_search/pi1/class.tx_kesearch_pi1.php:
00093:     {
00094:         $this->searchFormView = GeneralUtility::makeInstance('TYPO3\\CMS\\Fluid\\View\\StandaloneView');
00095:         $this->searchFormView->setTemplateRootPaths($this->conf['templateRootPaths']);
00096:         $this->searchFormView->setPartialRootPaths($this->conf['partialRootPaths']);
00097:         $this->searchFormView->setLayoutRootPaths($this->conf['layoutRootPaths']);

15 tx_kesearch_pi1::initFluidTemplate()

/var/www/web18/htdocs/relaunch2016_t3v8/typo3conf/ext/ke_search/pi1/class.tx_kesearch_pi1.php:
00067: 
00068:         // init template for pi1
00069:         $this->initFluidTemplate();
00070: 
00071:         // hook for initials

This goes on for a few more lines but I believe the problem lies in this part.

I've tried this solution but to no avail. As far as I can tell (using the Typo3 Object Browser) the entire config is not being loaded for ke_search at all.

This is (part of) the original typoscript for 7.6:

plugin.tx_kesearch_pi1 {
  templateRootPath = fileadmin/fluidtpl/_kesearch/Templates/
  partialRootPath = fileadmin/fluidtpl/_kesearch/Partials/
  layoutRootPath = fileadmin/fluidtpl/_kesearch/Layouts/
}
plugin.tx_kesearch_pi2 {
  templateRootPath = fileadmin/fluidtpl/_kesearch/Templates/
  partialRootPath = fileadmin/fluidtpl/_kesearch/Partials/
  layoutRootPath = fileadmin/fluidtpl/_kesearch/Layouts/
}

Using the solution from the previously mentioned answer:

plugin.tx_kesearch_pi1 {
    templateRootPaths {
        5 = EXT:ke_search/Resources/Private/Templates/
        }
    partialRootPaths {
        5 = EXT:ke_search/Resources/Private/Partials/
    }
    layoutRootPaths {
        5 = EXT:ke_search/Resources/Private/Layouts/
        }
}

plugin.tx_kesearch_pi2 {
    templateRootPaths {
        5 = EXT:ke_search/Resources/Private/Templates/
    }
    partialRootPaths{
        5 = EXT:ke_search/Resources/Private/Partials/
    }
    layoutRootPaths {
        5 = EXT:ke_search/Resources/Private/Layouts/
    }
}

I also tried adding the previously used templates as a fallback but it didn't seem to be recognized at all. I also cleared the cache after every step but nothing changed, still the same error.

I was prepared having to change the old template files to a certain degree but at this point I can't even get the default templates to work.

I found the mistake: I had another typoscript somewhere down the tree that had overridden the templateRootPath, partialRootPath and layoutRootPath settings. After removing this culprit, everything worked as intended.