I am a SCA and I "cleaned up" some SharePoint site permissions last week by removing membership groups from the Site Permissions list. This inadvertently caused everyone except Site Collection Admins to lose access to Classic Views. The users can still access pages and the content in Modern Views perfectly fine but if they try to access it in Classic View they get an access denied message. I created a test user account and assigned it Full Control permissions to replicate the problem on my end. Most of the site is still configured to utilize Class Views. Any suggestions how to fix this?
Finally determined what was causing the error. The Pages library did not have any permissions. I don't typically use this library but apparently permissions to it are necessary to view anything in Classic View.