grouped products not working

When I want to add or edit a grouped product in 2.5.5 it does not work.

### WordPress Environment ###

Home URL: https://x
Site URL: https://x
WC Version: 2.5.5
Log Directory Writable: ✔
WP Version: 4.5.2
WP Multisite: –
WP Memory Limit: 256 MB
WP Debug Mode: –
Language: nl_NL

### Server Environment ###

Server Info: Apache/2.4.10
PHP Version: 5.6.20-0+deb8u1
PHP Post Max Size: 22 MB
PHP Time Limit: 400
PHP Max Input Vars: 1000
SUHOSIN Installed: –
MySQL Version: 5.6.30
Max Upload Size: 22 MB
Default Timezone is UTC: ✔
fsockopen/cURL: ✔
SoapClient: ✔
DOMDocument: ✔
GZip: ✔
Multibyte String: ✔
Remote Post: ✔
Remote Get: ✔

### Database ###

WC Database Version: 2.5.5
:
woocommerce_sessions: ✔
woocommerce_api_keys: ✔
woocommerce_attribute_taxonomies: ✔
woocommerce_termmeta: ✔
woocommerce_downloadable_product_permissions: ✔
woocommerce_order_items: ✔
woocommerce_order_itemmeta: ✔
woocommerce_tax_rates: ✔
woocommerce_tax_rate_locations: ✔

### Active Plugins (5) ###

Acumulus: door SIEL Acumulus – 4.4.0
Advanced Code Editor: door BaInternet – 2.2.6
Akismet: door Automattic – 3.1.11
Advanced Automatic Updates: door pento – 1.0.2
WooCommerce: door WooThemes – 2.5.5

### Settings ###

Force SSL: –
Currency: EUR (€)
Currency Position: left
Thousand Separator: ,
Decimal Separator: .
Number of Decimals: 2

### API ###

API Enabled: ✔
API Version: 3.1.0

### WC Pages ###

Winkelbasis: #9583 - /winkel/
Winkelmand: #2 - /winkelmand/
Afrekenen: #3 - /afrekenen/
Mijn account: #4 - /mijn-account/

### Taxonomies ###

Product Types: external (external)
grouped (grouped)
simple (simple)
variable (variable)

### Theme ###

Name: AccessPress Store
Version: 2.0.6
Author URL: http://accesspressthemes.com/
Child Theme: ✕ – Als je wijzigingen aanbrengt in WooCommerce of in een hoofdthema dat je niet zelf gebouwd hebt
raden we het gebruik van een subthema aan. Zie: Hoe maak je een subthema

WooCommerce Support: ✔

### Templates ###

Overrides: accesspress-store/woocommerce/archive-product.php
accesspress-store/woocommerce/cart/cart.php
accesspress-store/woocommerce/cart/cross-sells.php
accesspress-store/woocommerce/content-product.php
accesspress-store/woocommerce/content-product_cat.php
accesspress-store/woocommerce/content-single-product.php
accesspress-store/woocommerce/global/breadcrumb.php
accesspress-store/woocommerce/global/sidebar.php
accesspress-store/woocommerce/global/wrapper-end.php
accesspress-store/woocommerce/global/wrapper-start.php
accesspress-store/woocommerce/loop/result-count.php
accesspress-store/woocommerce/myaccount/my-account.php
accesspress-store/woocommerce/single-product/meta.php
accesspress-store/woocommerce/single-product/product-image.php
accesspress-store/woocommerce/single-product/product-thumbnails.php
accesspress-store/woocommerce/single-product/related.php
accesspress-store/woocommerce/single-product/up-sells.php
accesspress-store/woocommerce/single-product.php

https://wordpress.org/plugins/woocommerce/