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If you have recently logged into the web site, and your phone app iOS or Android app reports 'Invalid credentials', then please log out, then log back in to your app again.
To this, when the app is open, select the 3 lines (burger button) top left, then scroll down the menu to the very bottom, until you see 'Sign Out'.
Select this, then the yellow button at the bottom of the screen to sign back in again.
I have added some guide images to the RHP Facebook and Twitter accounts to assist if this is not clear.