When saving a form, if it throws HTTP error, then it can be because of a number of things. It is evident that your server is throwing some sort of PHP related error. Overtime, we’ve seen the following are the most frequent ones.
php5-mcrypt library: It is a requirement of eForm. If it does not come installed in your server, then please get in touch with your host.
php memory limit: Occasionally, PHP would require more than 256MB of memory to process the large amount of variables. Try increasing to 512MB.
php max input var: The max_input_vars directive could be less in your server settings. Try increasing it.
server security: Some customized server security or server anti virus can prevent processing of large number of variables and inputs. Please consult with your webhost.