The synchronisation is very slowly.
To log in in an account, I need a code.
Today the code came too late. In the meantime I logged in on the PC.
What do you mean you need a code? K-9 doesn’t require any code and once your account is set up in K-9 you don’t need to log in again either.
So I don’t know where your issues comes from, but it can’t be K-9 
i suspect that the issue isn’t one of needing a code for K9, but rather an emailed 2fa code (from a 3rd party) is arriving too late for the OP to be able to use it to log into the 3rd-party site.
@UeliChristen – are you using POP or IMAP? if IMAP, make certain that the account in question is set for “push”, rather than “poll”, in the K9 app. that should get your mail synced in real time.
POP on K9 does not support push, and the lowest poll interval is 15min.
you should be able to do a manual refresh by swiping down on the mail list/directory screen of the inbox of the account where you are expecting the 2fa code.
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