Hey Jloupe!
I've been experiencing this issue for about a month now. One day, I was experimenting with some settings in WHMCS admin, then I changed something, then everything worked!
Instructions to fix bug:
Go to your WHMCS admin area
Go to Product/Services, then Product/Services again.
Select a package (any package)
Go to Module Settings
Scroll down and enable "Automatically setup the product when you manually accept a pending order"
Do this to ALL of your hosting plans