@Crenel84 You are correct, there was a change in 3.6.15 that causes this problem. The “0” value for confirmed is changed to an empty string. Depending on the mysql mode setting, trying to update the table with an empty string might be allowed or, as in your case, is rejected. My local phplist fails on this, but another installation is successful. That may be why it was not noticed.
See Php8fixes 202401 (#1026) · phpList/phplist3@f0443c3 · GitHub for the change to the file public_html/lists/admin/mysqli.inc
You can manually edit that file to remove the code change. I will submit a pull request to correct it properly.