Hello,
We use the original theme and the shopping card supplied with Prestashop.
We avoid the use of external modules to avoid any problems during update, we have very bad experience with that in the past.
Our IT administrator has updated our server (Debian 10 - PHP 7.3.26 - Maria DB 10.4.17) and installed the last Nginx version but nothing change.
We have try to deactivate the cache, but the problem is always present.
We must say also that we have a big database with around 10 000 products. When starting the backend, it take about 2 or 3 minutes before accessing the functions but we live with it. During this time, the frontend is also blocked. But that situation is not new for us, we know that since some years. So nothing to do with the related problem.
When examining the "informations" in the backend, we see these message: (sorry it's in french)
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vendor/composer/installers/src/Composer/Installers/TaoInstaller.php
vendor/composer/installers/src/Composer/Installers/Redaxo5Installer.php
vendor/composer/installers/src/Composer/Installers/MantisBTInstaller.php
vendor/composer/installers/src/Composer/Installers/KnownInstaller.php
vendor/composer/installers/src/Composer/Installers/DframeInstaller.php
vendor/composer/installers/src/Composer/Installers/SyliusInstaller.php