BPS standard simple troubleshooting & fixit steps: https://wordpress.org/support/topic/setup-steps-removal-steps-requirements-and-compatibility?replies=1
This appears to be a recent issue. Never happened in the past.
It’s probably going to be something specific to this particular Host server or something has changed or is different on this Host server if this is the same Host server. The htaccess code changes that have been made in previous BPS versions are not causing any problems in general/overall.
The first thing to eliminate would be: does this Host server allow locking the root .htaccess file.
http://forum.ait-pro.com/forums/topic/read-me-first-free/#root-htaccess-file-lock
If your Web Host does not allow locking of your Root .htaccess file/404 file permissions for your Root .htacces file, then go to the BPS htaccess File Editor tab page and click on the Turn Off AutoLock button. This will prevent your Root .htaccess file from being automatically locked when you upgrade BPS, which will prevent a 403 Error from occurring on your website.
Next thing to eliminate would be: does this Host allow using the “Options” htaccess directive.
http://forum.ait-pro.com/forums/topic/read-me-first-free/#internal-server-error
Some Web Hosts do not allow you to use the “Options” Directive in .htaccess files. If you see a 500 Internal Server Error then comment out the “Options” Directive by adding a pound sign (#) in front of Options -Indexes in your Root .htaccess file as shown below.
# DO NOT SHOW DIRECTORY LISTING
# If you are getting 500 Errors when activating BPS then comment out Options -Indexes
# by adding a # sign in front of it. If there is a typo anywhere in this file you will also see 500 errors.
#Options -Indexes
Some Web Hosts do not allow you to use the “DirectoryIndex” Directive in .htaccess files. If you see a 500 Internal Server Error then comment out the “DirectoryIndex” Directive by adding a pound sign (#) in front of DirectoryIndex in your Root .htaccess file as shown below. Known Hosts with this issue: NordNet
# DIRECTORY INDEX FORCE INDEX.PHP
# Use index.php as default directory index file
# index.html will be ignored will not load.
#DirectoryIndex index.php index.html /index.php
Next thing to eliminate is BPS Custom Code that is either not valid or is no longer allowed on a particular Host server.
http://forum.ait-pro.com/forums/topic/read-me-first-free/#500-error-custom-code
If you have added custom .htaccess code to BPS Custom Code, saved it, clicked the AutoMagic buttons and activated BulletProof Modes and there is either invalid .htaccess code in that custom .htaccess code or your particular Server/Host does not allow something in that custom .htaccess code then your site will crash with a 500 Internal Server Error.
1. Use FTP or your web host control panel file manager and delete your root .htaccess file (or the wp-admin .htaccess file if the custom .htaccess code was added to wp-admin Custom Code).
2. After you have deleted the .htaccess file or files, login to your site, go to BPS Custom Code, cut (NOT copy) the custom .htaccess code you added to any BPS Custom Code text boxes and paste that custom .htaccess code to a Notepad text file (use Notepad or Notepad++ – do NOT use Word or WordPad) and save it on your computer.
3. After cutting all custom .htaccess code that you have added to any BPS Custom Code text boxes, click the Save Root Custom Code button (and/or Save wp-admin Custom Code button), go to the Security Modes page, click the Create secure.htaccess File AutoMagic button and activate Root folder BulletProof Mode (and/or wp-admin Folder BulletProof Mode).
Your site should not crash at this point with a 500 Error. You can then check and test your custom .htaccess code individually. ONLY add one section of your custom .htaccess code at a time to BPS Custom Code text boxes (and do ALL the Custom Code steps) to isolate which custom .htaccess code is causing the 500 error. Either correct whatever needs to be corrected or just do not use the custom .htaccess code on your website if it does not work / is not allowed on your particular Host/Server.
