There is a workaround that allows you to sign with Norsk BankID Mobile.

If the Sign multiple documents with one signing key feature is elected, it is possible to identify with BankID Mobile, but when you get to the signing, this option will be turned off. It will look similar: 

What you then do to move forward and still sign with Norsk BankID Mobile is as follows: 

You start by pressing the one option that is Norsk BankID. You will then be taken to the page where you have to accept the "terms of content". Tick the box and press the arrow to continue: 

You will then be taken to the page where you must enter your SSN/User Identification number. Enter your SSN/User Identification number and press the arrow: 

You will now be taken to the page where you need to enter your One Time Code. Do not have to enter your code, but press Choose other BankID. Here it gives you the option to choose between BankID or BankID på mobil. Press BankID på mobil then One Time Code will change to BankID on mobile. You then press the arrow next: 

Once this is done, a reference code will be sent to your mobile. Then go to your phone and confirm that the same Reference is on your device as on your phone. 

What is irregular here, which does not happen when identifying with Norsk BankID Mobile, is that when you accept on the phone, you must then enter your Personal password after this is done and you have pressed the arrow. Then you have signed the documents.

If you have turned this feature on and run through the flow, this feature will be turned on for future transactions.