CiviCRM 5.6.1, WordPress 4.9.8, PHP 7.1.22, MySQL 5.7.23, WordFence 7.1.16
I just started getting this error when trying to create a mailing. It happens with Mosaico and traditional mailings. I previously saw this issue when first using the WordFence security plugin and found a discussion here suggesting to disable WordFence's XSS check, which I did with success until now. That was months ago. The option is still disabled and I even tried disabling WordFence entirely, but no luck. I haven't made any significant changes recently that I can think of. I had no problems with my last weekly mailing.
I also checked with my hosting provider to see if they had made any mod_security rules changes. They hadn't. I don't see anything interesting in the logs. Any ideas?