You should be able to fix that by adding this line to your constants.php
define( 'RECOVERY_MODE', TRUE );
If your site works after this, that would mean that one of your plugins/applications is causing the error.
If so, enable them one by one and check for the issue in a new tab for the same.
For me, it was an older version of Awards application causing the error. This means any of your plugins/applications which are incompatible to PHP 8 and IPS v4.7.3 are likely to cause such an error.
For more information about constants.php, please read this https://invisioncommunity.com/4guides/advanced-options/configuration-options/using-constantsphp-r25/