Hello Diogo and welcome back to the game and our new forums.
My name is Sofie and I will try to help you set a new nice avatar.
Since you are an experienced manager I can assume that you already know how we change our avatars on our game profile. Yes?
Please read the above topic for general guidelines of the new forums.
You have to change your Avatar on your game profile of your OSM account, and then automatically you will get the same avatar on your forum profile too.
Now, I need from you to start with a nice cleaning up of your browser's history, from your laptop or pc.
Just refresh your browser info by pressing CTRL+F5 . Or , clear all of your cache/temporary Internet Files (manually or using a software like CCleaner).
Also disable the use of any ad blockers on your browser.
Then try to loggin into your OSM account and I will remind you of the steps to set a new avatar, just to make sure everything will work fine mate.
I hope for a successful change Diogo. If nothing changes then we'll start talking of Bug reports mate.