Update :
I've just installed a CLEAN install of Joomla 4.2 in local with Wamp (PHP 8.0.13). Akeeba Backup is installed.
It seems to work fine but when I create a new page and click on save, it gives me a blank page.
But if I come back to the dashboard the page has been created.
I've tried a few things on a page, it looks ok but if I click on Save and Close, here it is : again the blank white page.
During the test I've downgraded the PHP version of Wamp to 7.4.26 but it still works ok. Also tried to uninstall SPPB pro and reinstall the lite version 3.8.9. Works as properly and upgrade to pro again : it's working with no problems (except for the blank pages that I mentioned before)
In the online website there are no extra components except for Akeeba Backup. I only had JEvents, I've uninstalled it but nothing has changed.
The online website is working on Joomla 4.2.3 and but PHP 7.4.30
I'll give you a superuser account for check