Actual behavior:
Page turns blank, with errors in the browser console. Screenshot attached below. Clicking back button in the browser does not load previous page. It navigates back through the navigation history without loading the pages. Nothing new is reported in the browser console.
There are no relevant logs in sonar.log or web.log
Thanks for reporting this issue. In fact, it has already been reported internally and a ticket has been created (SONAR-12591). This is scheduled to be fixed in the next release of SonarQube, v8.1.
Users with the global Administer permission should still be able to create new portfolios by accessing the global Administration > Configuration > Portfolios section of their instance.
If you click on the little arrow next to Configuration you should get a drop down with Portfolios as one of the options. Right now your selection is General Settings: