I've tested this now in three distinct farms with CentOS7.8 and the latest oVirt 4.3 release: OVA export files only contain an XML header and then lots of zeros where the disk images should be.

Where an 'ls -l <vmname>.ova' shows a file about the size of the disk, 'du -h <vmname>.ova' shows mere kilobytes, 'strings <vmname>.ova' dumps the XML and then nothing but repeating zeros until the end of the file.

Exporting existing VMs from a CentOS/RHEL 7 farm and importing them after a rebuild on CentOS/RHEL 8 would seem like a safe migration strategy, except when OVA export isn't working.

Please treat with priority!