See this post: https://www.phpbb.com/community/viewtopic.php?p=14681206#p14681206

TL;DR: The questionnaire is encoding RAW_DATA as base64 and we are sending that data via ajax to another server. Appearently some hosts are scanning the produced html code and if they recognize base64 in input field values, they might trigger mod_security and ban the admins that are calling the page.