Thanks to your log post, I was able to identify the proper place to find information about
the export process.
OVA exports don't fail for me, but result in empty disks, which is perhaps even
worse.
Two things I can suggest: Try with a new and smaller VM to see if it's actually a size
related issue or due to the fact that e.g. the number of disks in an OVA export are
limited.
The other is to make do with an export domain, which seems to work fine and might have
less issues with OVA file format constraints: Don't know if you just need the backup
or want to migrate...