multiple fatal errors in Apache2 log after update to Magento 2.4.3 & php 7.4

Question

Getting a lot of errors in the apache2 log after update. I am relatively new to Magento. Does this mean I need to troubleshoot one by one or I am missing one update that is causing multiple issues? Thank you for any hints.

[php7:error] [pid 24574] [client xx.xx.xx.xx:xxxxx] PHP Fatal error:  Uncaught Error: Using $this when not in object context in /var/www/html/vendor/magento/module-wishlist/view/frontend/templates/item/configure/addto.phtml:11\nStack trace:\n#0 {main}\n  thrown in /var/www/html/vendor/magento/module-wishlist/view/frontend/templates/item/configure/addto.phtml on line 11, referer: http://siccaro.eu/vendor/magento/module-wishlist/view/frontend/templates/item/configure/

[php7:error] [pid 24523] [client xx.xx.xx.xx:xxxxx] PHP Fatal error:  Uncaught Error: Call to a member function getData() on null in /var/www/html/vendor/magento/module-weee/view/frontend/templates/item/price/unit.phtml:13\nStack trace:\n#0 {main}\n  thrown in /var/www/html/vendor/magento/module-weee/view/frontend/templates/item/price/unit.phtml on line 13, referer: http://siccaro.eu/vendor/magento/module-weee/view/frontend/templates/item/price/

[php7:error] [pid 23929] [client xx.xx.xx.xx:xxxxx] PHP Fatal error:  Uncaught Error: Class 'PHPUnit\\Framework\\TestCase' not found in /var/www/html/vendor/magento/module-ui/Test/Unit/Component/MassAction/Columns/ColumnTest.php:17\nStack trace:\n#0 {main}\n  thrown in /var/www/html/vendor/magento/module-ui/Test/Unit/Component/MassAction/Columns/ColumnTest.php on line 17, referer: http://siccaro.eu/vendor/magento/module-ui/Test/Unit/Component/MassAction/Columns/

Answer 1

I haven't completely got rid of the errors, but I have made some progress by looking at the files that caused errors and I could see they were very old.

So these are my steps:

• fixing the directory permissions and ownership as per https://digitalstartup.co.uk/t/how-to-diagnose-and-fix-ownerships-and-permission-issues/211
• switching to developer mode (I don't know if it's crucial but I was tired of redeploying)
• deleting vendor subdirectories and composer.lock
• running composer install
• running magento upgrade
• switching to production mode

So now I mostly have one type of error left:

PHP Fatal error:  Uncaught Error: Class 'Magento\\Framework\\Component\\ComponentRegistrar' not found in

it is trying to find it in many differerent directories, but it is always that class...