Since I've migrated from WP 4.9.8 to 5.0, the text editor doesn't display TinyMCE buttons and controls (it become a simple textarea). I've installed the "Classic Editor" plugin (the official one for those who don't want to use Gutenberg), but the issue remains.
Could you help me ?
Well i believe that this issue will be fixed by wpForo asap, if verified.
Still, never update immediately when a version changes (4.x to 5 in this case). There is NO WAY not to have problems with plugins and WP.
There is no such issue in wpForo in general. This is a cache or some site specific issue. Please delete all caches, reset all optimizer/minyer plugins and press Ctrl+F5 on forum front-end. wpForo doesn't have any issue with WP 5.0 and with New Editor.The TinyMCE is available in WP 5.0 core so wpForo use it without any additional manipulation and without installing TinyMCE plugin, you can disable that plugin and use gutenberg without any problem.