I had this issue in old vBulletin 4 where some of the posts when edited, the editor was empty, no text inside.
Developer console (F12 key) shown no errors, only warnings.
Synchronous XMLHttpRequest on the main thread is deprecated because of its detrimental effects to the end userís experience. For more help http://xhr.spec.whatwg.org/ ckeditor.js:138:1411
I have checked PHP version and PHP module sin hosting control panel and seen no problem. Yet later I have found that the solution was to switch to older PHP version. Alternative solution was to tweak one file per: https://forum.unity.com/threads/vbulletin-editor-blanks-post-when-using-non-standard-characters-e-g-n-or.204088/
// make the following change to /vb/ckeditor.php
// Line 507:
// Change to:
htmlspecialchars($value, ENT_COMPAT | ENT_HTML401, "ISO-8859-1")
Was check when issue happened:
- other post were editable
- failing post re-posted again remains NOT editable
- Uninstalling addon (which means datastore cleanup?) not helps
- Disabling hooks in includes/config.php not helps
- Can not see error in error_log
- post SQL table column pagetext mediumtext latin1_swedish_ci Yes NULL

cashbb enabled: mysql/i -> nd_mysql/i; intl;
cashbb disabled: pgsql; pdo_pgsql; suhosin; tidy; timezonedb; uploadprogress; wddx; xmlrpc; zend_guard_loader; zip