After upgrading the Business Modeler from 5.9.1 to 5.11.0 I get a bunch of errors relating to select pages. The error message is 'This page can not be opened in content. It involves Select Pages out of a Reference Selector and from Reference Selector Add Button.
That's correct. It is no longer possible (as of 5.10.0) to show select pages in content:
Opening a select page in content is no longer allowed. This also applies to the "new" button in reference set selectors.