I had the same problem prevoiusly that my port 80n was blocked on my rounter since I normaly dont want access from the internet directly to my NAS. so I clicked several times for updating and after I noticed the still closed port, I was told that I have tried to many times.
Do you mean this error? There is a Failed Validation limit of 5 failures per account, per hostname, per hour. This limit is higher on our staging environment, so you can use that environment to debug connectivity problems. Exceeding the Failed Validations limit is reported with the error message too many failed authorizations recently.
I am not familiar with the Synology client for Lets Encrypt. But, if possible use an option for the staging test environment to avoid a repeat. Otherwise, just wait.