414 Request-URI Too Long

Yes, that's correct. The share URL was tested on latin chars, but I see it also includes non-latin multi-bit chars so it pass the limits.  We'll fix this in next version release, this is a post specific issue, it may work for  other share URLs without any problem.

If you need to fix this urgently please use Plugin File Editor ( Dashboard > Plugins > Edit ), or some File Manager. Open this file: /wp-content/plugins/wpforo/wpf-includes/class-api.php

Find this line:

$text = urlencode( wpforo_text( strip_shortcodes( strip_tags($text) ), 1000, false) );

Replace to this:

$text = urlencode( wpforo_text( strip_shortcodes( strip_tags($text) ), 200, false) );