Do the users you log in as have the appropriate permission set? 

Does the privileges error specify a specific record, resource, or item? There could be a field level security or sharing setting that’s keeping some part of the lead, account or opportunity pages private for this user. 

I believe they do. Any specific ones I should check?

This is exactly what it shows. “Insufficient Privileges. You do not have the level of access necessary to perform the operation you requested. Please contact the owner of the record or your administrator if access is necessary. For more information, see Insufficient Privileges Errors.”

Check the “Skuid Page Viewer” permission Set - your users need that.  This is assigned per user, not by profile.  You can configure Skuid to auto assign this permission set whenever a license is given to a user.  Also, double check that your users have access to the Visualforce pages that are calling your Skuid pages.

Chandra has shared some good suggestions above. Also, when you see the error, do you see any browser console errors too? Those can sometimes be more informative than what’s on the page itself.

I also wanted to link this conversation to the documentation that should help clarify what’s involved: https://docs.skuid.com/latest/en/skuid/deploy/salesforce/user-access/index.html

The profiles have access to the VisualForce page and have the skuid page viewer individually assigned but this still happens.

Do the affected users have anything in common, like their Role, sales team, or something like that?
Is this happening when they look at a specific lead, opportunity, or account in a detail page? Or, is this happening for list pages?

One possibility to look for is an uploaded file (image, document, or otherwise) that wasn’t shared publicly. I think if this is an issue, you would see an error in your browser console.
Another test you can run: if the users are running into this message when viewing a tab that’s been overriden by a Visualforce page, that’s a different scenario than if they were to view the direct link to preview the Skuid page. I would recommend finding the Skuid page that’s being used in this Visualforce page, then previewing it and then viewing that preview URL as the affected user. 

The users themselves can see all the pages. Admins cannot see these pages when logged in as the users. They share roles but when logged in as different people within these roles, admins can and cannot see the objects.

It sounds like it may be related to sharing rules, or it may be related to uploaded files that aren’t being shared publicly across the org. Have you checked to see if the sharing settings for the Account, Opportunity or Lead objects are in place to allow your own admin profile and the user’s profiles / roles to see them?

The sharing settings are public read/write and grant access using hierarchies.

I connected with Ogaga, and this error message is coming from another vendor’s installed package.