To resolve the issue, please use either Chrome, Edge or IE browsers. Firefox browser is having multiple issues with Core UI.
Note: in order to resolve IE11 Compatibility view error, please to the following:
- press ALT button once to bring IE11 menu
- click on Tools, Compatibility View settings
- uncheck "Display intranet sites in Compatibility View"
- make sure that Core address is not added to the list of websites there