How can I debug a Twig error that occurs after migrating from Drupal 8.9 to Drupal 9+

I am trying migrate my Drupal 8.9 site to Drupal 9+ and I am stuck with an error its cause I am not able to hunt down. On almost every page I get the following PHP exception:

Twig\Error\RuntimeError: The "replace" filter expects an array or 

"Traversable" as replace values, got "string" in "__string_template__faabd18c7a37a1e54d0ef10115b24022e72bc201187fff502de48bf58589321f" at line 2. in twig_replace_filter() (line 554 of /var/customers/webs/freigeist/www9/vendor/twig/twig/src/Extension/CoreExtension.php).

__TwigTemplate_75ee113db094d36d71f294529e9d7055b2065bf116719e9a04b7e4795be3121d->doDisplay() (Line: 405)
Twig\Template->displayWithErrorHandling() (Line: 378)
Twig\Template->display() (Line: 390)
Twig\Template->render() (Line: 45)
Twig\TemplateWrapper->render() (Line: 235)
Drupal\Core\Template\TwigEnvironment->renderInline() (Line: 52)
Drupal\Core\Render\Element\InlineTemplate::preRenderInlineTemplate()
call_user_func_array() (Line: 101)
Drupal\Core\Render\Renderer->doTrustedCallback() (Line: 786)
Drupal\Core\Render\Renderer->doCallback() (Line: 377)
Drupal\Core\Render\Renderer->doRender() (Line: 201)
Drupal\Core\Render\Renderer->render() (Line: 157)
Drupal\Core\Render\Renderer->Drupal\Core\Render\{closure}() (Line: 578)
Drupal\Core\Render\Renderer->executeInRenderContext() (Line: 158)
Drupal\Core\Render\Renderer->renderPlain() (Line: 419)
Drupal\views\Plugin\views\PluginBase->viewsTokenReplace() (Line: 1356)
Drupal\views\Plugin\views\field\FieldPluginBase->renderAltered() (Line: 1249)
Drupal\views\Plugin\views\field\FieldPluginBase->renderText() (Line: 1177)
Drupal\views\Plugin\views\field\FieldPluginBase->advancedRender() (Line: 238)
template_preprocess_views_view_field() (Line: 287)
Drupal\Core\Theme\ThemeManager->render() (Line: 436)
Drupal\Core\Render\Renderer->doRender() (Line: 201)
Drupal\Core\Render\Renderer->render() (Line: 1745)
Drupal\views\Plugin\views\field\FieldPluginBase->theme() (Line: 778)
Drupal\views\Plugin\views\style\StylePluginBase->elementPreRenderRow()
call_user_func_array() (Line: 101)
Drupal\Core\Render\Renderer->doTrustedCallback() (Line: 786)
Drupal\Core\Render\Renderer->doCallback() (Line: 377)
Drupal\Core\Render\Renderer->doRender() (Line: 201)
Drupal\Core\Render\Renderer->render() (Line: 717)
Drupal\views\Plugin\views\style\StylePluginBase->renderFields() (Line: 584)
Drupal\views\Plugin\views\style\StylePluginBase->renderGrouping() (Line: 475)
Drupal\views\Plugin\views\style\StylePluginBase->render() (Line: 2170)
Drupal\views\Plugin\views\display\DisplayPluginBase->render() (Line: 1533)
Drupal\views\ViewExecutable->render() (Line: 183)
Drupal\views\Plugin\views\display\Page->execute() (Line: 1630)
Drupal\views\ViewExecutable->executeDisplay() (Line: 81)
Drupal\views\Element\View::preRenderViewElement()
call_user_func_array() (Line: 101)
Drupal\Core\Render\Renderer->doTrustedCallback() (Line: 786)
Drupal\Core\Render\Renderer->doCallback() (Line: 377)
Drupal\Core\Render\Renderer->doRender() (Line: 201)
Drupal\Core\Render\Renderer->render() (Line: 241)
Drupal\Core\Render\MainContent\HtmlRenderer->Drupal\Core\Render\MainContent\{closure}() (Line: 578)
Drupal\Core\Render\Renderer->executeInRenderContext() (Line: 242)
Drupal\Core\Render\MainContent\HtmlRenderer->prepare() (Line: 132)
Drupal\Core\Render\MainContent\HtmlRenderer->renderResponse() (Line: 90)
Drupal\Core\EventSubscriber\MainContentViewSubscriber->onViewRenderArray()
call_user_func() (Line: 142)
Drupal\Component\EventDispatcher\ContainerAwareEventDispatcher->dispatch() (Line: 163)
Symfony\Component\HttpKernel\HttpKernel->handleRaw() (Line: 80)
Symfony\Component\HttpKernel\HttpKernel->handle() (Line: 57)
Drupal\Core\StackMiddleware\Session->handle() (Line: 47)
Drupal\Core\StackMiddleware\KernelPreHandle->handle() (Line: 191)
Drupal\page_cache\StackMiddleware\PageCache->fetch() (Line: 128)
Drupal\page_cache\StackMiddleware\PageCache->lookup() (Line: 82)
Drupal\page_cache\StackMiddleware\PageCache->handle() (Line: 47)
Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle() (Line: 52)
Drupal\Core\StackMiddleware\NegotiationMiddleware->handle() (Line: 23)
Stack\StackedHttpKernel->handle() (Line: 717)
Drupal\Core\DrupalKernel->handle() (Line: 19)

I found out that the error might be theme related (I am on Bootstrap). And on some pages I could narrow it down to the system block 'site_branding': Disabling this block makes the error disappear on some pages, but not on all.

My first guess was, that this might be an twig compatibility issue (may be in my bootstrap sub-theme), but it occurs also when I use other themes. So I am completely lost here.

How could I debug this error? Does the error message show, which component could be the cause of the error? Can anybody point out, which change between Drupal 8 and 9 could be responsible of the error?