Let's Encrypt certificate not issued due to error «error:06800097:asn1 encoding routines::string too long»

Symptoms

Let's Encrypt certificate is not issued, pending at the stage "Verification stage in a test environment".

Also errors of the following type are registered in the log at /usr/local/mgr5/var/letsencrypt.log:

May 15 15:40:01 [3178657:1] libmgr ESC[1;31mERROR Error: Type: 'crypto' Object: 'add_subject' Value: 'error:06800097:asn1 encoding routines::string too long'ESC[0m

Causes

The X509 encryption standard allows a maximum domain name length of 64 characters.

Solution

The domain name and its aliases in the certificate must contain no more than 64 characters each.