Exception #0 (InvalidArgumentException): Unable to serialize value. Error: Malformed UTF-8 character

[johnabs]

I ran into this issue when upgrading from 2.4.0 to 2.4.3. The issue is referenced here https://github.com/magento/magento2/issues/30486

"This issue comes because recaptcha module configuration values saved in core_config_data table which can't be serialised."

The fix is:

php bin/magento config:set recaptcha_frontend/type_recaptcha_v3/public_key <your_public_key>
php bin/magento config:set recaptcha_frontend/type_recaptcha_v3/private_key <your_private_key>
php bin/magento config:set recaptcha_backend/type_recaptcha_v3/public_key <your_public_key>
php bin/magento config:set recaptcha_backend/type_recaptcha_v3/private_key <your_private_key>
php bin/magento cache:flush

Hope it helps.