Okay, that explains... if you report yourself, you get a 404 (and that is what I tried). For anyone else, it's working fine for me. Are you sure that under the "User" field, you put the username of the person you want to report, and not of yourself?
Thank you, ProgramFOX. You were quite right! I wrongly thought that the form was asking to identify myself. Now I was able to write my complaint. And I hope too Lichess may give a satisfactory answer to the issue. Thanks again, people; I love Lichess!!
We do not give feedback, but rest assured that report will be examined and appropriate actions, where deemed so, will be taken.