Hello!
I just started a new webpage, and am trying to install Pro on it. The Pro license was used on another site previously as X-theme, but I revoked it and deleted the old website., and then upgraded to Pro cause it seemed pro (!).
I am using a fresh wordpress installation through Softaculous from my ISP. Everytime I install Pro, it seems to work fine, but only for less than a minute, and then I get 500-error on both url and wp-admin. It installs and validates fine. I’ve tried both newest and stable release. I’ve tried installing by uploading .zip in wp-admin, and I’ve tried manual install through ftp. I tried updating all permissions on folders and files to 755 and 644 according to guide.
The only way to get back to wp-admin, is to rename pro-folder in ftp to whatever, so wp automatically loads twentyseventeen or whatever.
Same issue arises every single time.
I activated debug mode, and first found an error in a .php file referring to a font-class-manager. That file had permissions 0000. Updating them to 644 didnt work, as it reset itself to 0000 when refreshing ftp-browser.
Other than that I have no idea. Please help.
EDIT:
I tried a new fresh install, and replicated the error-message from debug-mode:
Warning: require_once(/home/spilldsa/public_html/wp-content/themes/pro/cornerstone/includes/classes/styling/class-font-manager.php): failed to open stream: Permission denied in /home/spilldsa/public_html/wp-content/themes/pro/cornerstone/includes/cornerstone-plugin.php on line 105
Fatal error: require_once(): Failed opening required ‘/home/spilldsa/public_html/wp-content/themes/pro/cornerstone/includes/classes/styling/class-font-manager.php’ (include_path=’.:/opt/alt/php71/usr/share/pear’) in /home/spilldsa/public_html/wp-content/themes/pro/cornerstone/includes/cornerstone-plugin.php on line 105
EDIT2:
I managed to solve the problem after asking a friend and looking deeper in the file system. It turns out the following:
- class-font-manager.php is 10.47 kb, but empty of code in editor.
- class-font-manager.php has permissions 0000, and when changed to 0644, it automatically resets itself to 0000.
I unzipped the pro.zip locally, found class-font-manager.php, and copyed all the text, and pasted it in the empty class-font-manager.php in the ftp-file-system. Then:
- the permission-change to 0644 would stick, and page and wp-admin started working fine.
BUT THEN I updated from stable release 2.1.6 to newest release 2.2.5, and the same problem occured again. As expected, class-font-manager.php was empty, its permissions reset to 0000, and I had to copy in the contents from the file again.
Now everything is working again, but this is only a temporary solution. It seems that something is wrong somewhere else that I am not competent enough to figure out, where class-font-manager.php is corrupted upon installation or update (either manual or automatic).
Please respond and address this issue.
Sincerely, a tired person who has learnt a lot figuring out your bugs 
EDIT 3:
My ISP offered to do some digging, and after extensive research, they discovered that class-font-manager.php triggered a rule in the firewall and was immediately quarantined. They removed the quarantine, and white-listed the file. This should resolve future issues at update for my part, but it is still likely that others might experience the same issue!