There are a couple possible causes for this. First of all, check whether your mobile device has the correct time and date. There might be a small offset between your phone and the Topicus KeyHub server, so multiple tries might actually do the trick! Automatically updating these settings on your phone could help as well.
If this doesn't help and it has worked before, the best solution here is to request a reset of your 2FA-code. To do this you can click on the link that is shown on the same logon-screen where you enter the 2FA-code. After a Topicus KeyHub administrator approves your request, you will receive an email explaining what to do next.