I don’t know what’s wrong here. You should be able to migrate from GVM 11 to GVM 20.08. Maybe you can try to update gvmd to the latest 11 release version (should be 9.0.1 see GVM 11 (old stable, initial release 2019-10-14)) and do the migration afterwards.
Just to be sure you did import the old db using the same installation prefix and did run gvmd --migrate with the new gvmd?