I had empty text area field when tried to quick edit some post on vBulletin 4 forum.
Previously it was on PHP 5.3, now PHP 5.6 and new MySQL version + (MySQLi)
My posts had non standard characters in it.

I tried to disable all plugins and also open developer console of the web browser (F12).

Finaly i found this:


Line 507:

Changed to

htmlspecialchars($value, ENT_COMPAT | ENT_HTML401, "ISO-8859-1")
and that worked