Check if debug mode is on?

I have

'debug' => true,

but i got:

 This page is currently offline due to an unexpected error. We are very sorry for the inconvenience and will fix it as soon as possible.

Advice for developers and administrators:
Enable debug mode to get further information about the error. 

What can i do now?

Ok, when i delete this from my config

    'd4l' => [
      'static_site_generator' => [
        'endpoint' => NULL, # set to any string like 'generate-static-site' to use the built-in endpoint (necessary when using the blueprint field)
        'output_folder' => './static', # you can specify an absolute or relative path
        'preserve' => [], # preserve individual files / folders in the root level of the output folder (anything starting with "." is always preserved)
        'base_url' => '/', # if the static site is not mounted to the root folder of your domain, change accordingly here
        'skip_media' => false, # set to true to skip copying media files, e.g. when they are already on a CDN; combinable with 'preserve' => ['media']
        'skip_templates' => [], # ignore pages with given templates (home is always rendered)
        'custom_routes' => [] # see below for more information on custom routes
        'custom_filters' => [] # see below for more information on custom filters

the page is back. While the code is used in another demo-page, it should be ok.
Maybe this doesnt work with the theme zeroOne?

Yes, i should ask the author.

There is a comma missing before the last element of your list

should be:
'custom_routes' => [], # see below for