Update 2020-05-04: We've finally published Core and TKLDev as stable releases (v16.0); along with a few others. The RC releases noted here are no longer available for download. Please see the first instalment of updated appliances. Update 2020-05-15: Please see the second instalment of updated appliances.
As highlighted in the forums by Sean, Let's Encrypt recently discovered a bug in their Certificate Authority (aka CAA). The implication of this is that some of the certificates that they have issued, they shouldn't have!