The option of using two devices for authentication (e.g. an iPhone & iPad) only works on the online banking website of ApoBank. When using third-party apps that communicate with the bank via FinTS, such as Outbank, the above mentioned error message is transmitted by the bank server.
To solve the problem, deactivate one of your active TAN devices in your online banking. Afterwards you should be able to use Outbank again.
Please also note the information on the website of ApoBank: