You probably have an apple device and you may have just upgraded to IOS11?

• The problem is the local storage on your device is almost full and to save space your device is now deleting the cache each time you close the app.
• The local storage is where the app saves your login details for next time you want to use the app.
• This means that when you close the app the cache is emptied and the device forgets that you are logged in.
• We are working on a long term fix for this, but for the mean time you will have to free up some of the space on your device.

Please see: How to check the storage on your iPhone, iPad, and iPod touch