@tgw, please leave a list of all plugins too. And please let me know do you have built-in hosting server caching solutions?

I do not have any server solutions. I wrote this post after update to 1.5.0. All worked fine. After 2 days I updated forum to 1.5.1 and problem came back. Thanks to @tgw for notification.

I have found today 2 errors in log:

[Thu Jul 19 12:45:36.049904 2018] [lsapi:notice] [pid 860120:tid 140063329261312] [client ...] [host ] Backend log: PHP Warning:  array_filter() expects parameter 1 to be array, integer given in /home/user/domains/domain/private_html/wp-content/plugins/wpforo/wpf-includes/class-members.php on line 770\n, referer:  https://domain.com/wp-admin/user-edit.php?user_id=8450&wp_http_referer=%2Fwp-admin%2Fusers.php%3Fupdate%3Dadd%26id%3D8450 
[Thu Jul 19 12:45:36.065722 2018] [lsapi:notice] [pid 860120:tid 140063329261312] [client ...] [host ] Backend log: PHP Warning:  implode(): Invalid arguments passed in /home/user/domains/domain/private_html/wp-content/plugins/wpforo/wpf-includes/class-members.php on line 771\n, referer:  https://domain.com/wp-admin/user-edit.php?user_id=8450&wp_http_referer=%2Fwp-admin%2Fusers.php%3Fupdate%3Dadd%26id%3D8450 

I wrote this post after update to 1.5.0. All worked fine. After 2 days I updated forum to 1.5.1 and problem came back

That doesn't matter. I mentioned above when you update or re-install wpForo some caches are reset and you may think this is new version issue. but this is not a new version issue, this just a cache updating play.

We've tested this on dozens of websites during last few days and couldn't find such problem. So I think this is a 3rd plugin (caching plugin) or server caching issue.

Posted by: antonM

I have found today 2 errors in log:

[Thu Jul 19 12:45:36.049904 2018] [lsapi:notice] [pid 860120:tid 140063329261312] [client ...] [host ] Backend log: PHP Warning:  array_filter() expects parameter 1 to be array, integer given in /home/user/domains/domain/private_html/wp-content/plugins/wpforo/wpf-includes/class-members.php on line 770\n, referer:  https://domain.com/wp-admin/user-edit.php?user_id=8450&wp_http_referer=%2Fwp-admin%2Fusers.php%3Fupdate%3Dadd%26id%3D8450 
[Thu Jul 19 12:45:36.065722 2018] [lsapi:notice] [pid 860120:tid 140063329261312] [client ...] [host ] Backend log: PHP Warning:  implode(): Invalid arguments passed in /home/user/domains/domain/private_html/wp-content/plugins/wpforo/wpf-includes/class-members.php on line 771\n, referer:  https://domain.com/wp-admin/user-edit.php?user_id=8450&wp_http_referer=%2Fwp-admin%2Fusers.php%3Fupdate%3Dadd%26id%3D8450 

Could you please deactivate, delete wpForo, then search and install it again.