Plugin behavior on sub-domains

Hello,
I have a primary WordPress (current version) site at the top of my hosting directory but have added several sub-domains. One of them has a separate installation of WordPress, also the current version.

Recently my site was hacked and all databases became inaccessible. I was able to restore the two primary MySQL databases at the top of the hosting directory by using a restore point from a week earlier – no problem. However, I had to totally reinstall and rebuild the databases in the three sub-domains, including the one with the WordPress installation.

In reinstalling WordPress on the sub-domain I decided to add several security plugins: Wordfence and Bulletproof, to give me better access to backups and perhaps to better protect the site. They are both working well on my primary WordPress site at the top of the hosting directory, but installing those plugins caused the sub-domain site to become non-functional. I could not navigate past the first page without getting a 500 internal service error. Not sure what that is. Reinstalling (again) without security plugins solved the issue and the site is up and working well.

Is there a reason that adding a plugin to WordPress installed in a sub-domain would cause problems? I am now reluctant to add any plugins at all to the WordPress installed in the sub-domain.

Thanks for any insights!