Placing the search Block causes Error in Bootstrap (Sub)theme

If I try to place the Search Form Block in a region I get an error. Watchdog says:
LogicException: Render context is empty, because render() was called outside of a renderRoot() or renderPlain() call. Use renderPlain()/renderRoot() or #lazy_builder/#pre_render instead. in Drupal\Core\Render\Renderer->doRender() (Zeile 241 in /......./web/core/lib/Drupal/Core/Render/Renderer.php).