On the frontend- Warning Unable to find a user with the ID: 920 (every new user see it.)
Solved
No one can't signup even if the user registeration is enabled from the backend. When user want to sign up, the error is- CALL TO UNDEFINED METHOD JOOMLA\CMS\APPLICATION\SITEAPPLICATION::ISADMIN() [I have tried to see if the demo site on joomshaper is working. But i am surprised they have not any register link. This made me suspecious. Perhaps they already know it and this is why they have turned it off. Else without signup, what is the point of login?
This problem occurred for the acymailing. Now it has been disabled.
User registration works fine banckend and front end fine https://prnt.sc/ge3v3tlzBE--
Can't add user from the admin panel too. The error message is - Call to undefined method Joomla\CMS\Application\AdministratorApplication::isAdmin()
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Can't edit administrator profile on the backend. Error message is- Call to undefined method Joomla\CMS\Application\AdministratorApplication::isAdmin()
Check again
Can't move the hamburger menu to right corner. It stays on left to the logo always. no matter in which position you place.
Sorry it's not possible.
On mobile view, the pop up isn't responsive at all. it get cut from the top.
It works fine

If you somehow login as administrator in the frontend, you will get this message- "array_replace(): Argument #1 ($array) must be of type array, string given". However if you downgrade your php version to 7.4, you will see that the you can't log out. (Although if you select PHP 8.0 on their shop search page,
https://prnt.sc/ge3v3tlzBE--