Internal 500 Server Error

Actually my guess is that the problem is going to be with the root htaccess file. In PHP code you are always seeing the past after the initial first millisecond of php code processing is done. So it appeared that activating wp-admin BulletProof Mode caused the crash, but it was actually the root htaccess file. When you activated wp-admin BulletProof Mode you were seeing the present for a millisecond and not the past.

This statement you made confirms my guess.

There was no .htaccess file found in the wp-admin folder.

What type of hosting is this? Linux, Windows, something else?
Did you add any additional custom htaccess code to BPS Custom Code?
Are you using a caching plugin? If so, which one?

The server is Linux. Normally, I activate the default secure htaccess before adding custom code. This time I activated Zencache and copied the gzip code and pasted into custom field first, so that the first activation of root htaccess had the gzip code. Was that my problem? Not installing the default secure htaccess first? Puzzling to me, as it’s always been rock solid for me in the past. My main fear is having the problem repeat. If it happened again, any suggestions on how to fix without doing a restore? Thanks for your help. I really appreciate it.
david

For htaccess files you only need to do the troubleshooting steps below if there is a problem and would not have to do a restore. A 500 error when using htaccess files usually means either the server does not allow some of the htaccess code or there is a mistake with some of the htaccess code. ie one typo or invalid text/code in an htaccess code/files will cause a 500 error.

https://www.ads-software.com/support/topic/setup-steps-troubleshooting-steps-uninstall-steps-requirements-compatibility?replies=1

htaccess File Troubleshooting Steps: Unable to Login to Your Website:
1. Use FTP or your Web Host Control Panel File Manager and delete the .htaccess file in your website root folder and the .htaccess file in your wp-admin folder.
See Custom Code Note
2. Log into your website, click the BPS AutoMagic Buttons and Activate all BulletProof Modes. Or if you are removing BPS do steps 3 and 4 under BPS removal steps.

Thanks. I had deleted the root htaccess and replaced it with a vanilla WordPress htaccess and had same problem – but I had not attempted to login. I had contacted Bluehost tech support and was told that a PHP index to the theme (?) had been corrupted (way over my head…). I’ll mark this closed as it was mostly for my education. If it happens again and the steps to correct fail, I’ll open a fresh support ticket. Thanks for your time and patience in explaining this.
david

Just additional FYI info about WordPress theme template files for your reference.

https://codex.www.ads-software.com/Stepping_Into_Templates
The basic standard theme template files that are most commonly edited/changed are: index.php, header.php, footer.php, single.php, page.php, functions.php, comments.php, archive.php. There are other theme template files, but these are the ones that are most commonly edited/changed by folks.

Thank you very much. Your time in responding in this thread has been much appreciated. Support here has always been superb.
Regards,
david