Solve problems

Can I use several mobile apps on one mobile appliance?
I keep getting the message that my code is not valid. What should I do?
I would like to replace my smartphone or tablet. What happens now with the mobile app?
What do I need to log in using a mobile app?
What if I have several smartphones or tablets with the same mobile app?
Which mobile apps can I use to log in with 'one-time code via mobile app'?
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