"0" is an invalid render array key. Value should be an array but got a string. in Drupal\Core\Render\Element::children()

When upgrading from 9.5 to 10.3, rendering the home page returns the error below.

I've tried searching for the cause, but can't find it. The stacktrace mentions the theme, both the theme and maintenance theme i'm using are set in core.extensions theme. I've tried googling but the links I find seem to have slightly different issues and causes.

Any pointer would help greatly:).

InvalidArgumentException: "0" is an invalid render array key. Value should be an array but got a string. in Drupal\Core\Render\Element::children() (line 97 of core/lib/Drupal/Core/Render/Element.php).

Drupal\Core\Render\Renderer->doRender() (Line: 248)
Drupal\Core\Render\Renderer->render() (Line: 476)
Drupal\Core\Template\TwigExtension->escapeFilter() (Line: 65)
__TwigTemplate_5a57f8db7ac7ab73ebef615a50c49577->doDisplay() (Line: 393)
Twig\Template->yield() (Line: 349)
Twig\Template->display() (Line: 364)
Twig\Template->render() (Line: 35)
Twig\TemplateWrapper->render() (Line: 33)
twig_render_template() (Line: 348)
Drupal\Core\Theme\ThemeManager->render() (Line: 491)
Drupal\Core\Render\Renderer->doRender() (Line: 248)
Drupal\Core\Render\Renderer->render() (Line: 476)
Drupal\Core\Template\TwigExtension->escapeFilter() (Line: 329)
__TwigTemplate_1c10b7b5f11c822ef981cbc9e3007a19->doDisplay() (Line: 393)
Twig\Template->yield() (Line: 349)
Twig\Template->display() (Line: 364)
Twig\Template->render() (Line: 35)
Twig\TemplateWrapper->render() (Line: 33)
twig_render_template() (Line: 348)
Drupal\Core\Theme\ThemeManager->render() (Line: 491)
Drupal\Core\Render\Renderer->doRender() (Line: 248)
Drupal\Core\Render\Renderer->render() (Line: 476)
Drupal\Core\Template\TwigExtension->escapeFilter() (Line: 93)
__TwigTemplate_fdef4cbc091a25172c88e397d4f1ee92->doDisplay() (Line: 393)
Twig\Template->yield() (Line: 349)
Twig\Template->display() (Line: 364)
Twig\Template->render() (Line: 35)
Twig\TemplateWrapper->render() (Line: 33)
twig_render_template() (Line: 348)
Drupal\Core\Theme\ThemeManager->render() (Line: 491)
Drupal\Core\Render\Renderer->doRender() (Line: 248)
Drupal\Core\Render\Renderer->render() (Line: 158)
Drupal\Core\Render\MainContent\HtmlRenderer->Drupal\Core\Render\MainContent\{closure}() (Line: 638)
Drupal\Core\Render\Renderer->executeInRenderContext() (Line: 153)
Drupal\Core\Render\MainContent\HtmlRenderer->renderResponse() (Line: 90)
Drupal\Core\EventSubscriber\MainContentViewSubscriber->onViewRenderArray()
call_user_func() (Line: 111)
Drupal\Component\EventDispatcher\ContainerAwareEventDispatcher->dispatch() (Line: 186)
Symfony\Component\HttpKernel\HttpKernel->handleRaw() (Line: 76)
Symfony\Component\HttpKernel\HttpKernel->handle() (Line: 53)
Drupal\Core\StackMiddleware\Session->handle() (Line: 48)
Drupal\Core\StackMiddleware\KernelPreHandle->handle() (Line: 28)
Drupal\Core\StackMiddleware\ContentLength->handle() (Line: 32)
Drupal\big_pipe\StackMiddleware\ContentLength->handle() (Line: 106)
Drupal\page_cache\StackMiddleware\PageCache->pass() (Line: 85)
Drupal\page_cache\StackMiddleware\PageCache->handle() (Line: 48)
Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle() (Line: 51)
Drupal\Core\StackMiddleware\NegotiationMiddleware->handle() (Line: 36)
Drupal\Core\StackMiddleware\AjaxPageState->handle() (Line: 51)
Drupal\Core\StackMiddleware\StackedHttpKernel->handle() (Line: 741)
Drupal\Core\DrupalKernel->handle() (Line: 19)