Connection times out when trying to open a CFv6 form

I have a site that has 5 forms that were created using CFv6. The newest form is is a little bit larger than the others and has some custom PHP added to it, but is otherwise nothing out of the ordinary.

I can click on the first four and open them without the problem, however the fifth one times out and I get an error message: "Site temporarily unavailable. Connection timed out - please try again."

I've tried changing some possibly related PHP settings as follows:

php_value max_execution_time 60
php_value max_input_time 60
php_value memory_limit 256M

... however that did not fix the issue. What is the solution for this issue?

Anything in the console log? PHP logs?

No, neither. Those were the first things I checked. I set the error reporting to "maximum" -- nothing in the Joomla control panel. Also nothing in the browser console. Ditto the PHP logs.