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.

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.

How did this happen after the most recent update?
And why is this not failing for the other domain (they use the same user database)?

There is big work to do in the DB.

There are user ID's in wp_usermeta, which are having wrong capability or they are not included in wp_users table.

You would need to write lots of SQL, and other stuff, we can't provide the solution with writing the work step by step.

You would need a WordPress Dev with good WP knowledge, so the Dev could fix the issue.

Please be patient, need time to understand how we can help you without admin access.

I will come back to you after getting info from the Dev.

I am extremely hesitant providing anyone admin acces to my website - I hope you can understand that.

This problem is related to your DB, we will need Admin Access to your website.

Send the credentials to infogvectors.com, mention this topic in the email, and let me know here that the email is sent.

Thanks

Error in members.php:

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

I'm running my website in 2 languages and utilize WP Multisite for that.

Interesting observation:
On one site I do not get this error (Tweaking4all.com - English language) and
on the other site it crashes my entire website (Tweaking4all.nl - Dutch language).

Both domains are on the share the same setup, running (all add-ons licensed and activated):

- Wordpress Multisite 6.1.1
- wpForo Version 2.1.7 (active)
- wpForo Ads Manager Version 3.0.2(active)
- wpForo Advanced Attachments Version 3.0.4(active)
- wpForo Embeds Version 3.0.3(active)

Any help or suggestions would be appreciated since one of my sites is currently down due to this issue.