Cannot migrate a WordPress site from PHP 7.4 to 8

All Plug-ins are updated. WordPress is version 6.1.1.

When I update to PHP 8, the website works just fine but I cannot log in into WordPress Dashboard.

It just shows me: array_filter(): Argument #1 ($array) must be of type array, bool given.

Any tips? Should we start to look for PHP developer or it is something that we can fix ourselves?

We are using Custom Theme and would prefer not to contact the developer.

I will appreciate any help. Thank you.


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