We found the reason of this problem. This is the database table charset problem. In this case your database table charset is utf8_general_ci.

This is an old charset, current version of WordPress and wpForo requires utf8mb4_unicode_ci charset.

SOLUTION

Make sure your hosting service MySQL version is 5.5.3 and higher. You can check it in Dashboard > Forums > Dashboard admin page.

To check the charset please open your website database manager (phpMyAdmin) and check the charset of your forum tables. In Your website hosting service > cPanel > phpMyAdmin tool open the database table list and check, the right charset is utf8mb4_unicode_ci or utf8mb4_unicode_520_ci:

You should ask your hosting service to change all tables with _wpforo_ prefix to the mentioned charset.

In case you want to do that yourself, just execute this SQL queries in Hosting Service cPanel > phpMyAdmin database manager, navigate to SQL Tab. If your WordPress table prefix is not wp_   replace all wp_ prefixes in this code to your database table prefixes, if they are wp_ just use it as is:

SET SQL_MODE = "NO_AUTO_VALUE_ON_ZERO";
ALTER TABLE `wp_wpforo_forums` CONVERT TO CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci;
ALTER TABLE `wp_wpforo_languages` CONVERT TO CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci;
ALTER TABLE `wp_wpforo_likes` CONVERT TO CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci;
ALTER TABLE `wp_wpforo_phrases` CONVERT TO CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci;
ALTER TABLE `wp_wpforo_posts` CONVERT TO CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci;
ALTER TABLE `wp_wpforo_profiles` CONVERT TO CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci;
ALTER TABLE `wp_wpforo_topics` CONVERT TO CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci;
ALTER TABLE `wp_wpforo_usergroups` CONVERT TO CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci;
ALTER TABLE `wp_wpforo_views` CONVERT TO CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci;
ALTER TABLE `wp_wpforo_visits` CONVERT TO CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci;
ALTER TABLE `wp_wpforo_votes` CONVERT TO CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci;

If you have wpForo Private Messages addon, it's recommended to do the same for PM tables:

SET SQL_MODE = "NO_AUTO_VALUE_ON_ZERO";
ALTER TABLE `wp_wpforo_pmfolders` CONVERT TO CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci;
ALTER TABLE `wp_wpforo_pms` CONVERT TO CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci;