XF 2.1 Error Importing theme

I'm getting the following while trying to import a theme.

"Oops! We ran into some problems. Please try again later. More error details may be in the browser console."

Screenshot (15).webp
 
We had the same issue on a different host, and they wouldn't help us so my clients switched to OVH host and set up an Apache Server there. What needs to be done to fix this "mod security" issue?
Just checked it, it is returning a 403 error code, which I'm sure you know is Forbidden. So what steps does my client need to take to resolve said possible issue with mod security?
 
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