Hi Alex, I have been unable to reproduce this error on both a fresh buster install as well as an upgraded stretch.
According to the release notes[1], the min size has been raised to 2048bit, but not 4k. Please check if your /etc/ssl/openssl.cnf requires longer keys. Best regards, Markus [1] https://www.debian.org/releases/stable/amd64/release-notes/ch-information.en.html#openssl-defaults
