I got rid of the errors by clearing out the certificates folder at caddy home e.g. rm -rf /var/lib/caddy/.local/share/caddy/* and then restarting. No errors since.
I guess some zerossl remnants were laying around in certificates…
BTW: the only way I found that was thank to this comment
Would be nice to add a note to Conventions — Caddy Documentation something along the lines of “remember $HOME is /var/lib/caddy for the caddy user. when run as a service.” That took a while to figure out…