The error seems specific to SF Apex class ‘AppSettingsPageController'.
You can check your profile permissions for Apex classes in Salesforce by following these steps:
Go to Setup – Click the gear icon in the top right corner and select Setup.
Find Profiles – In the Quick Find box, type Profiles and select Profiles from the results. (If you don’t know your profile. Go to Users, search for your name, and check your profile there.)
Choose Your Profile – Locate and click the profile you want to check.
Check Enabled Apex Class Access – Scroll down to the Apex Class Access section. Here, you'll see a list of classes the profile has access to.
Modify Access if Needed – If you need to add or remove access to a class, click Edit, select the relevant classes, and save your changes.
FAQ -
Why does this not occur in PRD even when the profile isn’t assigned to the Apex class?
This could be due to differences in the SF environment.
Even though the class was added to the profile in the affected Gold environment, other environments like PRD may have different permission sets or profiles that inherently grant access to the Apex class without the need to explicitly add it to the profile.
To investigate further on exact cause, recommend comparing the profile settings, permission sets, and security configurations across environments.