Check you don’t have any firewalls or security features on your new server or CDN. This error can occur when they cause a save request to be rejected because they incorrectly think it contains bad content.
Thanks for the tip @Kevin. I vaguely remember having this issue years ago when I setup my old site. For the life of me I can’t figure it out though. I’m running through Cloudflare, so I’m 99% certain this is the issue. Site is hosted on Digital Ocean with a similar droplet to old site, so unlikely there is something there.
Will keep digging and ask around Cloudflare forums. Thanks again.