Various iOS apps log out randomly, lose all data & settings
A found a 'satisfactory' workaround. As I commented on the question, I was having the bug after I decided to close all applications using the app switcher and then start every application on my phone.

My 'solution' is to close all applications in the app switcher and then turn off the device. Then, turn on the phone. 

Since that, I have not had problems.