That might match up with either of two different errors from the Let's Encrypt CA:
The main limit is Certificates per Registered Domain (50 per week). A registered domain is, generally speaking, the part of the domain you purchased from your domain name registrar. For instance, in the name
www.example.com , the registered domain is
example.com . In
new.blog.example.co.uk , the registered domain is
example.co.uk . We use the Public Suffix List to calculate the registered domain. Exceeding the Certificates Per Registered Domain limit is reported with the error message
too many certificates already issued , possibly with additional details.
[...] a Duplicate Certificate limit of 5 per week. [...] Exceeding the Duplicate Certificate limit is reported with the error message
too many certificates already issued for exact set of domains .
That is, it could be because of other people getting 50 certificates this week for other domain names under
ddns.net (which seems very possible), or because of you trying successfully 5 different times to get a certificate for
Is there any information in the DSM interface, or from your own recollection of what you did, that could help decide which it is?
In the first case (other people's activity), you might have to pick a different free domain name provider, wait a while and try again, register your own domain, or ask the ddns.net operators to try to get in touch with Let's Encrypt about the rate limits.
In the second case (your own activity), you should look for logs and documentation that will help make sure that you don't make the same certificate request over and over again. In this case you might have to wait for a week before trying again.
Based on this search
I think it's much more likely to be due to other people's behavior (reaching a rate limit for the shared research of the ddns.net domain name) than to your own actions, since it doesn't look like you successfully issued any certificates for this name recently.