Error setup letsencrypt on nginx

[Domsday2017]

I have successfully installed directadmin on my vps. Then I proceed to create the domain and subdomain for it
Now I want to give letsencrypt certificate for this domain and subdomain

I followed the instructions of the page https://help.directadmin.com/item.php?id=648

But when I click the Save button in SSL Certificates returns an error.
The error message is as follows:

Cannot Execute Your Request

Details

Getting challenge for xxx.net from acme-server...
Waiting for domain verification...
Challenge is valid.
Getting challenge for sub01.xxx.net from acme-server...
Waiting for domain verification...
Challenge is valid.
Getting challenge for www.xxx.net from acme-server...
Waiting for domain verification...
Challenge is valid.
Generating 4096 bit RSA key for xxx.net...
openssl genrsa 4096 > "/usr/local/directadmin/data/users/admin/domains/xxx.net.key.new"
Generating RSA private key, 4096 bit long modulus
..............................++
............................++
e is 65537 (0x10001)
[B]Size of certificate response is smaller than 500 characters, it means something went wrong. Printing response...[/B]

Please tell me the cause and how to fix the error. I really need it
I would like to provide information about the directadmin that I am using. Sorry my english very bad.

[root@vps custombuild]# ./build versions
Latest version of DirectAdmin: 1.51.3
Installed version of DirectAdmin: 1.50.1

DirectAdmin 1.50.1 to 1.51.3 update is available.

Latest version of Let's Encrypt client: 1.0.10
Installed version of Let's Encrypt client: 1.0.10

Latest version of Nginx: 1.10.3
Installed version of Nginx: 1.10.3