White Screen when preview child theme

Hi,
I’ve set up a child theme for graphene which worked well on 3 other sites that I’m running. On the stated site however the preview shows a WSOD and do not dare to activate it ??
Here’s the rawdebug code:

{
  "parnt": {
    "deps": [
      [],
      []
    ],
    "signals": {
      "failure": 0,
      "thm_noqueue": 1,
      "err_php": 1,
      "thm_notheme": 1
    },
    "queue": [],
    "irreg": [],
    "swaps": [],
    "url": "https://marketstrategy.de/?ModPagespeed=off&ao_noptimize=1&preview_ctc=1&template=graphene&stylesheet=graphene&now=1593354490"
  },
  "child": {
    "deps": [
      [],
      []
    ],
    "signals": {
      "failure": 0,
      "thm_noqueue": 1,
      "err_php": 1,
      "thm_notheme": 1
    },
    "queue": [],
    "irreg": [],
    "swaps": [],
    "url": "https://marketstrategy.de/?ModPagespeed=off&ao_noptimize=1&preview_ctc=1&template=graphene&stylesheet=graphene-child&now=1593354490"
  }
}

This is what I get from WP_DEBUG:

Notice:  Undefined property: FDX_Widget_1::$plugin_slug in /usr/www/users/abiria/marketstrategy.de/public_html/wp-content/plugins/wp-mobile-edition/admin/class-widgets.php on line 9

Notice:  Undefined property: FDX_Widget_1::$plugin_slug in /usr/www/users/abiria/marketstrategy.de/public_html/wp-content/plugins/wp-mobile-edition/admin/class-widgets.php on line 9

Warning:  session_start(): Cannot start session when headers already sent in /usr/www/users/abiria/marketstrategy.de/public_html/wp-content/plugins/ip2location-country-blocker/ip2location-country-blocker.php on line 2181

Warning:  Cannot modify header information - headers already sent by (output started at /usr/www/users/abiria/marketstrategy.de/public_html/wp-content/plugins/wp-mobile-edition/admin/class-widgets.php:9) in /usr/www/users/abiria/marketstrategy.de/public_html/wp-content/plugins/ip2location-country-blocker/ip2location-country-blocker.php on line 1420

Warning:  Cannot modify header information - headers already sent by (output started at /usr/www/users/abiria/marketstrategy.de/public_html/wp-content/plugins/wp-mobile-edition/admin/class-widgets.php:9) in /usr/www/users/abiria/marketstrategy.de/public_html/wp-content/plugins/ip2location-country-blocker/ip2location-country-blocker.php on line 1421

Warning:  Cannot modify header information - headers already sent by (output started at /usr/www/users/abiria/marketstrategy.de/public_html/wp-content/plugins/wp-mobile-edition/admin/class-widgets.php:9) in /usr/www/users/abiria/marketstrategy.de/public_html/wp-content/plugins/ip2location-country-blocker/ip2location-country-blocker.php on line 1422

Warning:  Cannot modify header information - headers already sent by (output started at /usr/www/users/abiria/marketstrategy.de/public_html/wp-content/plugins/wp-mobile-edition/admin/class-widgets.php:9) in /usr/www/users/abiria/marketstrategy.de/public_html/wp-content/plugins/ip2location-country-blocker/ip2location-country-blocker.php on line 1423

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