Today I upgraded php from 7.0 to 7.2 on my Linode server using Laravel Forge. Super easy: click a button in Forge, and it will upgrade it. But then sometimes there are issues. I reloaded my site to be greeted with error 500's everywhere. So I quickly set debug to true (probably nobody looks at this site anyway, so it doesn't much matter), and found out that there was a permission error for the app/storage/logs/laravel.log file.

The exact error was Error in exception handler: The stream or file "/app/storage/logs/laravel.log" could not be opened: failed to open stream: Permission denied in [...]

So, if you find yourself in this situation, it's pretty easy to fix.

• SSH into your server.
  ssh {youruser}@{yourip_here}

• cd {yourappname}

• Note: Never set a directory to 777. Not even on dev. Just get used to doing it the proper way: change directory ownership instead. So set your current user that you are logged in with as owner and the webserver user (www-data, apache, ...) as the group.
  sudo chown -R {your_user}:www-data storage
sudo chown -R {your_user}:www-data bootstrap/cache

• Then set directory permissions
  chmod -R 775 storage
chmod -R 775 bootstrap/cache

So there you go, that should fix it.