Opening a K2 Form in SharePoint shows a blank page. This issue can be intermittent or only occur for specific users. User can have read permissions on the site and full control on the item they are trying to open.
We have found that when Publishing is used, the Browse Directory permission is also required. This permission is not enabled on Read's permission levels. "Browse Directories" permission is needed; otherwise, the Form is not displayed correctly (or not at all).