iphone/ipad/android visualization problem

Hi all,
when I try to view my virtual tour on an iPad I see some grey areas (see the attached image). I got the same problem on my Android phone. On my desktop PC everithing works fine.

I'm not sure about that, but I suppose the areas appear where the front-face and the under-face meet each other. Do you have any idea on how to fix it?

Thank you very much for your valuable support.

Thanks for your reply, I'll try to upload my tiles again...hope something is broken

Great, the problem was in the uploaded tiles.

Thank you so much.

Best regards.