Fatal error after WordPress update

Hello everyone,

the website was a pretty old wordpress platform, so we decide to update it.
Right after the automatic update, I got this message:
Briefly unavailable for scheduled maintenance. Check back in a minute.

Following the guide here: https://codex.www.ads-software.com/FAQ_Troubleshooting#How_to_clear_the_.22Briefly_unavailable_for_scheduled_maintenance.22_message_after_doing_automatic_upgrade.3F
I removed the .maintenance file, and after that I got this fatal error:
Fatal error: Cannot redeclare _http_build_query() (previously declared in /home/v3hz86ha/public_html/wp-includes/compat.php:18) in /home/v3hz86ha/public_html/wp-includes/functions.php on line 721

Note: I backed-up the files and database before I did the update.

Any ideas how to fix this and to run the website properly?
Thanks in advance.