@Malinois wrote:
This earned me around 6GB of storage space, which is now apparently suddenly gone.
Dropbox doesn't remove bonus space earned through referrals. The only time that would happen is if you abused the referral system.
Dropbox is now asking me to pay for the storage space that I have earned by inviting new users.
No, they're likely asking you to pay because you're over quota. Dropbox doesn't charge for space earned through referrals.
Go to your Plan page and check your space usage. Provide a screenshot if you can, being sure to mask any private information. It would look like this: