Additional: There is another topic - same problem - Rate limit error
And there is a Letsencrypt rate limit:
You can create a maximum of 10 Accounts per IP Address per 3 hours. You can create a maximum of 500 Accounts per IP Range within an IPv6 /48 per 3 hours. Hitting either account rate limit is very rare, and we recommend that large integrators prefer a design using one account for many customers.
So if Sslforfree creates a new account per user, this limit is hitted.
But that's only a problem of that tool, not a general Letsencrypt limit.
Select your own client.