[RSS Feed/News] Error message when trying to put board in debug mode

[XenForo]

When trying to enable debug mode I get this error on every page:

Code:

An exception occurred: [ErrorException] [E_DEPRECATED] Return type of XF\Session\Session::offsetExists($key) should either be compatible with ArrayAccess::offsetExists(mixed $offset): bool, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice in src/XF/Session/Session.php on line 175

Any ideas?
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