In most cases, 2FA fails because the time on each device is not synchronized. For 2FA to function properly, the date and time on the device on which you are logging in to Proton Mail must be exactly the same as those of the device where you receive your 2FA code.
If the time and date mismatch, you will receive the “Incorrect login credentials” message.
If you have synced your devices, but you are still unable to access your account, please make sure that you have followed the step-by-step instructions for enabling 2FA in this article(new window).
You can also check common login problems(new window) and their solutions.