What are you trying to submit in your fields. In the few cases I’ve seen this happen it was when trying to submit JavaScript in a field, don’t remember what plugin did it.
I don’t know anything about ModSecurity, but I’d guess that there is some setting that needs to be changed. There’s really nothing in ACF. They do have support and if this was a problem for me that’s where I’d be looking https://modsecurity.org/help.html
