Thank you for your quick reply !
The error message is not a browser message (it is a woocommerce notice), and is in french but it translates more or less like this :
“This order cannot be paid. Please contact us if you need assistance”
I realize a plugin evolved with a cache option that I didn’t notice, I just added cart and order paths to urls excluded from cache. We’ll see if this resolves the problem but thank you for your advice !