There is a known iOS problem that some app data such as the login is deleted if the device has very little memory on the iPhone or iPad. So it helps to optimize the memory, i.e. to delete or swap out unused apps and to store photos/videos in the iCloud. The following guide from Apple should help: https://support.apple.com/en-gb/HT201656 


An update to solve the problem is already in development. We therefore ask for a little patience. Thanks a lot.