As some may have noticed, the Dutch forum took a nose dive and crashed the Dutch website constantly.
Producing this error (posting the error, just in case someone is looking for this specific error):

Warning: Attempt to read property "roles" on bool in /home/user/public_html/wp-content/plugins/wpforo/classes/Members.php on line 1108

Fatal error: Uncaught Error: Attempt to assign property "roles" on bool in /home/user/public_html/wp-content/plugins/wpforo/classes/Members.php:1108 Stack trace: #0 /home/user/public_html/wp-content/plugins/wpforo/classes/Members.php(1166): wpforo\classes\Members->synchronize_user('2', Array) #1 /home/user/public_html/wp-content/plugins/wpforo/includes/installation.php(32): wpforo\classes\Members->synchronize_users(100) #2 /home/user/public_html/wp-content/plugins/wpforo/classes/Actions.php(20): wpforo_activation() #3 /home/user/public_html/wp-includes/class-wp-hook.php(308): wpforo\classes\Actions->wpforo\classes\{closure}('') #4 /home/user/public_html/wp-includes/class-wp-hook.php(332): WP_Hook->apply_filters('', Array) #5 /home/user/public_html/wp-includes/plugin.php(517): WP_Hook->do_action(Array) #6 /home/user/public_html/wp-content/plugins/wpforo/wpforo.php(279): do_action('wpforo_after_in...') #7 /home/user/public_html/wp-content/plugins/wpforo/wpforo.php(510): wpforo\wpforo->init_classes() #8 /home/user/public_html/wp-includes/class-wp-hook.php(308): wpforo\wpforo->init(Object(WP)) #9 /home/user/public_html/wp-includes/class-wp-hook.php(332): WP_Hook->apply_filters(NULL, Array) #10 /home/user/public_html/wp-includes/plugin.php(565): WP_Hook->do_action(Array) #11 /home/user/public_html/wp-includes/class-wp.php(797): do_action_ref_array('wp', Array) #12 /home/user/public_html/wp-includes/functions.php(1332): WP->main('') #13 /home/user/public_html/wp-blog-header.php(16): wp() #14 /home/user/public_html/index.php(17): require('/home/user/...') #15 {main} thrown in /home/user/public_html/wp-content/plugins/wpforo/classes/Members.php on line 1108

It appears that the forum doesn't like dangling meta data in the wp_usermeta table.

This is how I fixed the issue - wasn't that hard once you know where to look.

1. Made a backup of the tables wp_usermeta and wp_users. Just in case.

2. Determine what records in wp_usermeta is no longer linked to an existing user (NULL).

SELECT *
FROM wp_usermeta LEFT OUTER JOIN wp_users on wp_usermeta.user_id=wp_users.ID
WHERE wp_users.ID IS NULL

3. If all looks good, remove these rows 

DELETE wp_usermeta
FROM wp_usermeta LEFT OUTER JOIN wp_users on wp_usermeta.user_id=wp_users.ID  
WHERE wp_users.ID IS NULL

4. Test and verify if everything works as it should.