Player having problems to see map and token's layers

I made a technical test with one of my players before starting my new game. It seems he can't see the map and the tokens I set up. everything looks black. He can see the 3d dice rolling, he can see the playing card I use for my game, the chat, the character sheet, but no map or token. I tried to modify various options, to show or hide portions of the map, to disable fog of war from the general options for my game .... nothing. I need hints to understand If I must keep looking into my settings as a master, or if the problems are due to some issue in my player's setting/usage. I tried to login as a player and I see maps and tokens regularly

Have you checked to see if the token has its "Has Sight" option checked?  Sometimes when a token is created, a GM (me) can update the "has sight" but forget to re-set the token on the character sheet so the next time the GM (me) drags it out, it's the player token from before the GM (me) set the "has sight" option. 

thanks I checked fog of war and vision options, but after various attempts we found that the problem was generated by my player's browser, he changed it and everything was resolved.