My clients can't pay with Paypal

[Quishr]

Hello, 

First of all I hope I've posted my message in the right category.
I'm having a big issue for few months now. My clients can't pay with Paypal Express Checkout installed on my Shopify store. After I choose Paypal solution and connect with my customer's account, there's always an error message saying "We can't complete your payment for the moment. This transaction was denied for regulatory compliance reasons." (approximative translation from french)

I have no clue what does it mean and Paypal support seems not being able to solve it. Did someone had this issue and knows how to solve it ? Thank you community. 

Bests