WordPress Core is causing 500 internal server error


We are facing this error:

PHP Fatal error:  Uncaught Error: Call to undefined function get_locale() in /var/www/html/wp-includes/l10n.php:137
Stack trace:
#0 /var/www/html/wp-includes/l10n.php(828): determine_locale()
#1 /var/www/html/wp-includes/class-wp-fatal-error-handler.php(46): load_default_textdomain()
#2 (internal function): WP_Fatal_Error_Handler->handle()
#3 {main}
  thrown in /var/www/html/wp-includes/l10n.php on line 137

The error occurs since the major update to 5.5, or 5.5.1 respectively. A hotfix we found is replacing the l10n.php file on the webserver with the freshly downloaded one from wordpress, but after some time (roughly a day) the error will come up again. This happened several times in a row now. Things tested:

  • Disabling all Plugins
  • Change Theme
  • Manually update WordPress Core
  • Installing Wordfence

Can this error be caused by a plugin conflict or a theme error, and then maintain while the plugins are disabled and theme changed?