It might be a good idea to compile more specific information about this as a Wiki. someone will need it.
I also had a problem issuing certificates with ZeroSSL a while ago. When I log in to my account I see the following message:
Note: Your account is holding more than 100 ACME certificates. For using the Dashboard or API with this amount of ACME certificates you have to upgrade at least to the ZeroSSL basic plan.
Related topics have been posted in the past:
ZeroSSL doesn’t mention it prominently, but it was recently secretly added to this page
(Perhaps this is their strategy to encourage you to purchase a paid plan…):
Note
In case you have more than 100 ACME certificates you need at least a ZeroSSL basic plan in order to work with those in Dashboard or API. In case you have more than 100K ACME certificates you need at least a ZeroSSL premium plan in order to work with those in Dashboard or API.
and ZeroSSL may be experiencing problems with the ACME issuance process as the number of users increases.
So I had to revert to Let’s Encrypt (=LE). As a result, (LE rate limits were somewhat relaxed at that time)
some subdomains (free domains, For example FreeDNS) were suspended.
Note: If your domain uses many subdomains and you encounter LE rate limiting, I recommend applying for LE’s rate limiting form. In many cases, LE will approve this and extend the limit.
I learned that other ACMEs have recently increased, and I was also able to get a VPS to test them. I recently tested the Byepass certificate. Some subdomains were restored at that time.