I have a large form that I am submitting via an ajax call. The form is dynamic and can vary in size. I use the serialize function to gather the data and it is working fine, but when I get past a certain number of variables, I get a 403 forbidden error. I have set my max_input_vars to 20000 and my post_max_size to 4096M. Both of these appear in a .user.ini file. Any ideas why this error would appear past a certain size or number of variables?
Check the error log. A form size, should not cause a 403, but would truncate the data if there was a limitation.