I have discovered an issue with how uln-yum-mirror works with Oracle Linux 8 repos. With Oracle Linux 8 there is a new concept called Application Streams (AppStreams) and with that on the yum/dnf side there's some metadata involved. A new file called "modules.yaml.gz".
You can see this on one of the public-yum repos here:
https://public-yum.oracle.com/repo/OracleLinux/OL8/appstream/x86_64/repodata/
On our local yum mirror, the file is downloaded during the sync process to:
/var/www/html/yum/OracleLinux/OL8/appstream/x86_64/modules.yaml.gz
While the uln-yum-mirror process is currently broken with regards to this because it does not add this metadata to the repomd.xml, I have a workaround. You can use the 'modifyrepo' command to manually add the metadata to the repo like this:
# cd /var/www/html/yum/OracleLinux/OL8/appstream/x86_64
# modifyrepo --simple-md-filenames ./modules.yaml.gz repodata/