Hi @Patmo,
The error that you saw is basically associated with a Let's Encrypt client that crashed. In that case, it can leave server-side resources in an occupied state. This can last for quite some time even when the renewal is not being re-attempted frequently.
There has been a server-side change very recently (today) that treats this situation a bit more leniently:
If the new change doesn't fix your issuance problem, we would really need to see the earlier client-side logs from /var/log/letsencrypt to understand how and why your client crashed in the process of requesting certificates (probably failed automated renewals). There is a known bug where this can happen sometimes when you use --standalone with a TCP port that's unavailable because that port is bound by a running webserver.