The ERROR: The number of NVTs in the OpenVAS Manager database is too low.
persisit even after openvasmd --rebuild.
Your database is likely too low if for some reason the sync scripts failed. Running greenbone-ntv-sync --rsync, greenbone-cert-sync and greenbone-scap-sync should fix your problem.
If not, then you may have a firewall / network issues along the path preventing the feeds updates to finish properly.