It's possible that a user has signed out on his/her device.
If so, the user has to try signing in again. If signing in fails to work, an error message will appear Invalid credentials.
The following steps need to be checked if this occurs. 

  1. Make sure that the device has a strong 3G, 4G or Wifi-connection. 
  2. Make sure the username and password are spelled correctly. Perhaps @ or has been forgotten, the name might have been misspelled or the e-mail address and/or password do not start with a capital letter. 
  3. It's possible that the account is linked too many groups or groups that are too large. This can be checked in the dashboard. If this is the case, the app will not be able to load the large amount of data and as a consequence it will stall when       signing in. 
  4. Check in Google Play Store or in the Google App store if there is a new update available for the app. Perhaps the device is running on an old version of the app. Always make sure that the latest version is installed. 
  5. Close the app, start the app and try again to login.
  6. If signing in doesn't work after having tried all of the above solutions, try delete and reinstall the app.