Frontend pages are blank when a user is logged in

When I'm logged in to the site, all pages on the front-end of the site are blank and there is an Internal Server Error in the browser console.

In the debug log I see the following errors:

PHP Fatal error: [] operator not supported for strings in ***/wp-content/plugins/siteorigin-panels/siteorigin-panels.php on line 363
PHP Fatal error: [] operator not supported for strings in ***/wp-content/themes/flash-pro/inc/extras.php on line 37

I've tried to deactivate all plugins and switch to the default WordPress theme, but the issue happens with this theme as well:
PHP Fatal error: [] operator not supported for strings in ***/wp-content/themes/twentyseventeen/inc/template-functions.php on line 34

I've tried to use different PHP versions(5.5, 5.6, 7.0), but no luck.