Hello and please excuse me if I am speaking in layman's terms here. I too wanted to increase the PHP version from 7.4 to 8.0; under Joomla 3 X and Helix Ultimate this was ultimately no problem with a current SP Page Builder.
After switching to Joomla 4, I also get the error message
count(): Argument #1 ($value) must be of type Countable|array, null given
in connection with the module "Login Form
Without logging in, everything works; with logging in, too, at first - but at the latest after logging out, the error message appears.
With a downgrade to PHP 7.4 the page runs again.
After some cross-reading, I see that the problem is not unknown; however, I cannot see whether there is already a solution for this? Especially one that the hobby user can implement?
Thank you for relevant tips
Christian
Translated with www.DeepL.com/Translator (free version)