Forad

Надеюсь будет полезно, внезапно появилась точно такая же проблема.
При отключении ModSecurity ошибка пропала.
Вот ответ хостинга:
I have investigated this issue further for you and it seems that the reason for the same is caused due to the fact that your CMS application is uploading the image via ajax call and uploading it directly on the page. This on the other hand is trigerring one of our custom ModSecurity rules and returning a 406 error during the upload.