Why do I keep getting asked for the Passphrase of the Master Key?
Android automatically quits background tasks if you are low on memory space. This might affect a process which is responsible for keeping the passphrase present in memory. You must re-enter your passphrase when starting up Threema. Threema cannot change this.
If you are frequently asked to provide your passphrase, we recommend you to disable it and use global encryption on your device instead. You can find this in your phone's settings: Security > Encrypt phone or Security > Encrypt device.