After trying to post my problem on discuss.flarum.org without success (I couldn’t get the confirmation e-mail from flarum to my gmail account so I could activate my flarum account in order to post messages), I decided to try again the process of purging the HTML head items extension, cleaned the forum cache and my browser history, and then the residual part of the Google Analytics tag finally disappeared, so probably when you logged into the forum, it was already gone.
Sorry for not coming back to give you an update right away, I didn’t think you were going to further investigate the issue. My bad, and I apologize for it.
PS: I will leave the fftest admin account in place, since it might be helpful in the future. I will try to add Google Analytics again, and give you an update about my success or failure.
Thank you again for taking your time to help me, you are a true gentleman!