err_empty_response: due to wrong configuration?

Hello! First of all, some bg information:

Background information:

Main domain: https://domain.com (standalone site that will be deleted after development site is completed)
Development site: https://domain.com/temp (completely separate from domain.com; shares different WP admin panel from domain.com; multisite installation with same top level domain: 1 main site, 2 subsites)

Recently, after updating some of my plugins and WordPress itself, I ran into the infamous white screen of death issue.

Based on the debugging report, it seems like the reason was caused by 2 plugins, AAM and WPML String Translation.

Also, based on my hosting service provider, they discovered that I have some messed up core WordPress files on both my main site and my development site and they also mentioned that my CPU % load is very high.

Hence, wp-admin and wp-includes were both replaced with fresh and latest ones. The sites then appear back to normal and I can access the dashboard again.

Then on the development site (the multisite installation), I deactivated all my plugins, installed and network activated Server Stats and proceed to reactivate the rest of my plugins one by one and the following issues happened which my hosting provider said could be partly due to Server Stats:

Issue:

1. I keep getting this: err_empty_response when I try to update or save my pages on Google Chrome
2. The error (using Firefox browser) is:

Secure Connection Failed

An error occurred during a connection to sens.com.sg.

The page you are trying to view cannot be shown because the authenticity of the received data could not be verified.
Please contact the website owners to inform them of this problem.

Report from hosting provider:

1. First of all, there was a high number of requests to your sites wp-login pages, which can indicate a brute force attack

– wp-login.php requests per site:
73 sens.com.sg
144 sens.com.sg-ssl_log

2. In addition, number of requests to default WordPress cron job is following:

– wp-cron.php requests per site:
244 sens.com.sg-ssl_log

3. Also a huge amount of time is spent for the WordPress Heartbeat API:

– heartbeat (admin-ajax.php) requests per site:
2532 sens.com.sg-ssl_log

Are there any configurations in my Server Stat settings that I should tweak in order to correct this? Or are these issues totally unrelated to Server Stats?

Thank you!