I tested this on a fresh Joomla 4 install with Acymailing acymailing_enterprise_v8.7.1. So, this is not related to any plugin, template override, etc.
The issue is, when I create a new menu item of type "Acymailing >> User Profie", it gives this issue:
Class "acym_JFormField" not found
I then did a search in the entire package, and I found that "back >> libraries >> joomla >> fields.php" has a blank class "acym_JFormField" with no code in it. Looks like Acymailing team forgot to include code inside this class.
Could somebody look into it with urgency?