I believe this error message is the Draytek software misinterpreting the error condition from Let's Encrypt.
In November 2019, the "old" version of the Let's Encrypt API was disabled for new registrations. You can find more information on the topic here: End of Life Plan for ACMEv1 .
If the Draytek software has not been updated to use the new Let's Encrypt ACMEv2 API, then that part of the router's functionality will no longer work.
This answer is speculative, but I think given the error message and that you mention November specifically, it is a likely explanation.