The issue is observed after upgrade to SCS v3.1.49. Information on dashboard is visible but the assets (css, image, icons, etc) can not be loaded, "sync mirror" button cat not be loaded neither.
Spring Cloud Services v3.1.49
The issue is fixed in SCS v3.1.50 and above releases. As a workaround, git mirrors can be refreshed by using CF CLI, or API endpoint.