If you are logged out from your device, with the message "You have logged in on another device", then please make sure that noone else is using your account. This message means that your credentials might be used by someone else as well on other devices.

As your sessions' security is top priority for us, we will make sure that you are notified by such security threats, and by changing your credentials, you can immediately solve such vulnerabilities. Please also note that your credentials are not stored on our servers, only its derived hash, moreover, all of your data is encrypted with a hash, that is not even known by us, only your client devices, so attackers have no means of getting readable data from us.
You can read more about our security solutions.

If you are intentionally sharing your account however, please check out our term of use, that clearly prohibits doing so. One account is intended for one user.