Ok… apparently, the icon problem was happening at several computers and browsers EVERYTIME the icon set was changed.
It can be fixed with CONTROL+F5 (reload plus delete browser cache)
But if I fix it and then change the icon set again, the same icon problem returns until a new CONTROL+F5
While this can seen very simple at first glance, not every user will even know about “control+F5”. Even more power users types will take a time to realize that.