Upgrade to php 8.x

Dear all, My website works fine until I upgrade the server to php 8.0.
Have unabled the plugins and the theme, without success. Equally updated core files of WP. I seem to get the error once I log in wp-admin:

[12-Dec-2025 23:00:04 UTC] PHP Fatal error: Uncaught Error: Call to undefined function filter_block_content() in /home/forge/test-aeolus.on-forge.com/publ>
Stack trace: 0 /home/forge/test-aeolus.on-forge.com/public/wp-includes/class-wp-hook.php(341): wp_pre_kses_block_attributes() 1 /home/forge/test-aeolus.on-forge.com/public/wp-includes/plugin.php(205): WP_Hook->apply_filters() 2 /home/forge/test-aeolus.on-forge.com/public/wp-includes/kses.php(1159): apply_filters() 3 /home/forge/test-aeolus.on-forge.com/public/wp-includes/kses.php(965): wp_kses_hook() 4 /home/forge/test-aeolus.on-forge.com/public/wp-includes/functions.php(6124): wp_kses() 5 /home/forge/test-aeolus.on-forge.com/public/wp-includes/functions.php(6071): wp_trigger_error() 6 /home/forge/test-aeolus.on-forge.com/public/wp-includes/kses.php(929): _doing_it_wrong() 7 /home/forge/test-aeolus.on-forge.com/public/wp-settings.php(239): require(‘/home/forge/tes…’) 8 /home/forge/test-aeolus.on-forge.com/public/wp-config.php(149): require_once(‘/home/forg in /home/forge/test-aeolus.on-forge.com/public/wp-includes/form>

[12-Dec-2025 23:00:04 UTC] PHP Fatal error: Uncaught Error: Call to a member function set() on null in /home/forge/test-aeolus.on-forge.com/public/wp-incl>
Stack trace: 0 /home/forge/test-aeolus.on-forge.com/public/wp-includes/l10n.php(960): load_textdomain() 1 /home/forge/test-aeolus.on-forge.com/public/wp-includes/class-wp-fatal-error-handler.php(49): load_default_textdomain() 2 [internal function]: WP_Fatal_Error_Handler->handle() 3 {main}

thrown in /home/forge/test-aeolus.on-forge.com/public/wp-includes/l10n.php on line 857

any advice would be welcome! Thanks!

Stefaan

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