It has now happened tice that my Apache 2.4.41 (on Ubuntu 20.04) automatically performs a graceful restart around midnight, but the previously running website doesn't "come back". The relevant entries in /var/log/apache/error.log are
AH00171: Graceful restart requested, doing restart [:error] [pid 443511] avahi_entry_group_add_service_strlst("XXXXX") failed: Not supported [:error] [pid 443511] avahi_entry_group_add_service_strlst("XXXXX") failed: Not supported
XXXXX is a valid fqdn under the
.de top level domain (and I can only guess that the line occurs twice because it is once for http and once for https).
I don't know what really happens then: If I try a graceful restart by hand, it works without any problems. Searching for the "avahi_entry_group_add_service_strlst" message, I find only cases with other reasons, e.g. "Invalid host name" or "local name collision", but never "Not supported". Even less do I have an idea what could be the cause.