According to the technicians of the apoBank, this error appears when the apoTAN+ app of the bank hasn't been activated properly. 

To use the apoTAN+ app, you need an activation letter from the bank and go through a few activation steps.
There are also instructions on the website:
https://www.apobank.de/unsere-leistungen/konto-karte/mobile-banking/apotan-plus

If you didn't receive any activation letter from the bank, contact the apoBank customer service directly via phone +49 211 5998 8000 and ask for help.

Please also be aware of following restrictions: 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.

In case you added to TAN-devices, you need to deactivate one of them on the online banking website to be able to use Outbank.