Apache2 on Debian 9 and 10 is started from systemd. it has
PrivateTmp=true
PrivateTmp=true
in it'sits definition. So processes started from the server don't have access to the regular /tmp and /var/tmp directories. Instead, they have their own directories.
You can either move the socket out of /tmp or start apache2 with
PrivateTmp=false
PrivateTmp=false