What is happening is:

1. I visit my limesurvey site, it works.
2. I modify config.php. Note: privileges do not change when this is done, they remain drwxrwx--- that is, 770.
3. Now, I go to visit my site, and I get an Internal Server 500 error response.

How could this be?

If I follow the steps in the post I made here  (also shown below), then the site works again. Is this a result of caching and doing a cache check of some sort?

1. Run
2. Visit the site.

3. Run

You didn't mention what you changed in config.php and what version of LimeSurvey is used. Your last thread about that topic is nine months old. What changed?

We know nothing about the webserver and the php handler (dso, php-fcgi, php-fpm etc.). A plain CentOS distribution or is a controlpanel installed?
Is SELinux active?

When having 500er errors you usually take a look at logfiles of webserver and php handler. Currently this looks like an issue unrelated to LimeSurvey.