css not loading on Apple Devices – possibly because of domain mapping

Hello,
I have a weird problem which might be linked to domain mapping.

I have a premium theme which creates some css files in the same folder as the images. This css was not loading on some devices / browsers. I tried to open the css directly in the browser (link provided) and I can’t open it on those devices / browsers.
Devices / browsers with problem :
– iPad, safari / firefox / chrome
– iPhone, safari
– macbook, safari
– samsung tablet, samsung browser
Devices / browsers with no problem :
– windows laptop, firefox / edge / safari / chrome
– samsung tablet, firefox

If I enter the non-mapped path, the file opens properly on all devices.
nb : I deactivated all the plugings but WordPress MU Domain Mapping, it didn’t make a difference.

If someone has a clue that would be most appreciated.
Thanks in advance,
Magali

The page I need help with: [log in to see the link]