The Selectivor app is designed to keep you logged in for your convenience. The main reason you may be logged out is an issue with internet connection. When your internet connection is weak or doesn't exist, you may see an "Error!" message or be automatically logged out.
Rest assured - the app is functioning fine! Be sure you have internet connection before opening Selectivor. Also adjust any phone/internet settings that may create intermittent loss of connectivity, such as having wifi-only connection options.