There is already a bug report here on the GitHub page of keepassxc. Reverting to 2.3.3 fix the issue.
This is the line in the code that displays the warning.
I’m posting it here since it seems there is some trouble identifying the root cause of the problem, maybe a library missing or a mismatch qt version?
Here the list of new commits in 2.3.4
I’m using Ubuntu 18.04. Thanks.