Ok, I have no quick solution, but I infer there was just an update to Thunderbird? If true, then the update may have lost your profile. My suggestion to pursue that would be to click help>troubleshootinginformation, scroll down to 'profiles' and click 'about:profiles'. That takes you to a pane showing available profiles. One of them may be your profile prior to the upgrade.

what I get is the folowing: see attachement

Let's start over. It appears I was wrong; you are trying to set up an account on Thunderbird? Please translate the error message to English to ensure several people here can read the issue. Thank you.

indeed, I am trying to set up an account on Thunderbird. the error message says : registration not possible, username, configuration or password not correct.

I suggest not inserting password. The configuration can complete without it and you will be prompted later for password. My guess is that may be the only issue, as everything else looks proper. If that goes correctly, the next issue will be to find the password. Some email providers now require separate password when used on an email client. Examples include Gmail, Verizon, AOL, AT&T and possibly others, so that may be true for Mailfence. But I don't know.