Magento 2.3.2 – DOMXPath Exception in Backend After Upgrade from 2.2.7

yesterday, I upgraded a multistore Magento 2.2.7 to 2.3.2 (that was a real headache to get it done) and now I am facing a really weird problem.

If I navigate (in backend) to Stores > Configuration, the following error is thrown and I have no clue, why this happens:

1 exception(s): Exception #0 (Exception): Warning: DOMXPath::query():
Invalid expression in /www/vendor/magento/framework/Config/Dom.php on
line 336

Exception #0 (Exception): Warning: DOMXPath::query(): Invalid
expression in /www/vendor/magento/framework/Config/Dom.php on line 336

#1 DOMXPath->query('') called at [vendor/magento/framework/Config/Dom.php:336]
#2 Magento\Framework\Config\Dom->_getMatchedNode('') called at [vendor/magento/framework/Config/Dom.php:216]
#3 Magento\Framework\Config\Dom->_mergeNode(&DOMElement#0000000029ee61af000000005b530d30#, '') called at [vendor/magento/framework/Config/Dom.php:149]
#4 Magento\Framework\Config\Dom->merge('<?xml version="1...') called at [vendor/magento/module-config/Model/Config/Structure/Reader.php:97]
#5 Magento\Config\Model\Config\Structure\Reader->_readFiles(&Magento\Framework\Config\FileIterator#0000000029ee61cc000000005b530d30#) called at [vendor/magento/framework/Config/Reader/Filesystem.php:132]
#6 Magento\Framework\Config\Reader\Filesystem->read('adminhtml') called at [vendor/magento/framework/Config/Data/Scoped.php:119]
#7 Magento\Framework\Config\Data\Scoped->_loadScopedData() called at [vendor/magento/framework/Config/Data/Scoped.php:97]
#8 Magento\Framework\Config\Data\Scoped->get(NULL, NULL) called at [generated/code/Magento/Config/Model/Config/Structure/Data/Interceptor.php:37]
#9 Magento\Config\Model\Config\Structure\Data\Interceptor->get() called at [vendor/magento/module-config/Model/Config/Structure.php:127]
#10 Magento\Config\Model\Config\Structure->__construct(&Magento\Config\Model\Config\Structure\Data\Interceptor#0000000029ee61d0000000005b530d30#, &Magento\Config\Model\Config\Structure\Element\Iterator\Tab#0000000029ee61c2000000005b530d30#, &Magento\Config\Model\Config\Structure\Element\FlyweightFactory#0000000029ee61cd000000005b530d30#, &Magento\Config\Model\Config\ScopeDefiner#0000000029ee61ce000000005b530d30#) called at [generated/code/Magento/Config/Model/Config/Structure/Interceptor.php:14]
#11 Magento\Config\Model\Config\Structure\Interceptor->__construct(&Magento\Config\Model\Config\Structure\Data\Interceptor#0000000029ee61d0000000005b530d30#, &Magento\Config\Model\Config\Structure\Element\Iterator\Tab#0000000029ee61c2000000005b530d30#, &Magento\Config\Model\Config\Structure\Element\FlyweightFactory#0000000029ee61cd000000005b530d30#, &Magento\Config\Model\Config\ScopeDefiner#0000000029ee61ce000000005b530d30#) called at [vendor/magento/framework/ObjectManager/Factory/AbstractFactory.php:116]
#12 Magento\Framework\ObjectManager\Factory\AbstractFactory->createObject('Magento\Config\M...', array(&Magento\Config\Model\Config\Structure\Data\Interceptor#0000000029ee61d0000000005b530d30#, &Magento\Config\Model\Config\Structure\Element\Iterator\Tab#0000000029ee61c2000000005b530d30#, &Magento\Config\Model\Config\Structure\Element\FlyweightFactory#0000000029ee61cd000000005b530d30#, &Magento\Config\Model\Config\ScopeDefiner#0000000029ee61ce000000005b530d30#)) called at [vendor/magento/framework/ObjectManager/Factory/Dynamic/Developer.php:66]
#13 Magento\Framework\ObjectManager\Factory\Dynamic\Developer->create('Magento\Config\M...') called at [vendor/magento/framework/ObjectManager/ObjectManager.php:70]
#14 Magento\Framework\ObjectManager\ObjectManager->get('Magento\Config\M...') called at [vendor/magento/framework/ObjectManager/Factory/AbstractFactory.php:160]
#15 Magento\Framework\ObjectManager\Factory\AbstractFactory->resolveArgument(array('instance' => 'Magento\Config\M...'), 'Magento\Config\M...', NULL, 'configStructure', 'Magento\Config\C...') called at [vendor/magento/framework/ObjectManager/Factory/AbstractFactory.php:246]
#16 Magento\Framework\ObjectManager\Factory\AbstractFactory->resolveArgumentsInRuntime('Magento\Config\C...', array(array('context', 'Magento\Backend\...', true, NULL), array('configStructure', 'Magento\Config\M...', true, NULL), array('sectionChecker', 'Magento\Config\C...', true, NULL), array('backendConfig', 'Magento\Config\M...', true, NULL), array('resultForwardFac...', 'Magento\Backend\...', true, NULL)), array()) called at [vendor/magento/framework/ObjectManager/Factory/Dynamic/Developer.php:34]
#17 Magento\Framework\ObjectManager\Factory\Dynamic\Developer->_resolveArguments('Magento\Config\C...', array(array('context', 'Magento\Backend\...', true, NULL), array('configStructure', 'Magento\Config\M...', true, NULL), array('sectionChecker', 'Magento\Config\C...', true, NULL), array('backendConfig', 'Magento\Config\M...', true, NULL), array('resultForwardFac...', 'Magento\Backend\...', true, NULL)), array()) called at [vendor/magento/framework/ObjectManager/Factory/Dynamic/Developer.php:59]
#18 Magento\Framework\ObjectManager\Factory\Dynamic\Developer->create('Magento\Config\C...', array()) called at [vendor/magento/framework/ObjectManager/ObjectManager.php:56]
#19 Magento\Framework\ObjectManager\ObjectManager->create('Magento\Config\C...') called at [vendor/magento/framework/App/ActionFactory.php:44]
#20 Magento\Framework\App\ActionFactory->create('Magento\Config\C...') called at [vendor/magento/framework/App/Router/Base.php:306]
#21 Magento\Framework\App\Router\Base->matchAction(&Magento\Framework\App\Request\Http#0000000029ee62c9000000005b530d30#, array('areaFrontName' => '_admin', 'moduleFrontName' => 'admin', 'actionPath' => 'system_config', 'actionName' => 'index', 'variables' => array('key' => 'bd60ea3c35a3a120...'))) called at [vendor/magento/framework/App/Router/Base.php:167]
#22 Magento\Framework\App\Router\Base->match(&Magento\Framework\App\Request\Http#0000000029ee62c9000000005b530d30#) called at [vendor/magento/framework/App/FrontController.php:95]
#23 Magento\Framework\App\FrontController->dispatch(&Magento\Framework\App\Request\Http#0000000029ee62c9000000005b530d30#) called at [vendor/magento/framework/Interception/Interceptor.php:58]
#24 Magento\Framework\App\FrontController\Interceptor->___callParent('dispatch', array(&Magento\Framework\App\Request\Http#0000000029ee62c9000000005b530d30#)) called at [vendor/magento/framework/Interception/Interceptor.php:138]
#25 Magento\Framework\App\FrontController\Interceptor->Magento\Framework\Interception\{closure}(&Magento\Framework\App\Request\Http#0000000029ee62c9000000005b530d30#) called at [vendor/magento/framework/Interception/Interceptor.php:153]
#26 Magento\Framework\App\FrontController\Interceptor->___callPlugins('dispatch', array(&Magento\Framework\App\Request\Http#0000000029ee62c9000000005b530d30#), array(array('default_store_se...', 'page_cache_from_...', 'storeCookieValid...', 'install', 'configHash'))) called at [generated/code/Magento/Framework/App/FrontController/Interceptor.php:26]
#27 Magento\Framework\App\FrontController\Interceptor->dispatch(&Magento\Framework\App\Request\Http#0000000029ee62c9000000005b530d30#) called at [vendor/magento/framework/App/Http.php:137]
#28 Magento\Framework\App\Http->launch() called at [generated/code/Magento/Framework/App/Http/Interceptor.php:24]
#29 Magento\Framework\App\Http\Interceptor->launch() called at [vendor/magento/framework/App/Bootstrap.php:261]
#30 Magento\Framework\App\Bootstrap->run(&Magento\Framework\App\Http\Interceptor#0000000029ee62bb000000005b530d30#) called at [index.php:39]

Did anyone face the same situation and knows what to do right here?
I just don't see the important thing in this error message.