I have had similar issues and found clearing the redis cache helped.
refer >> https://www.mail-archive.com/openvas-discuss@wald.intevation.org/msg11222.html 67
Command will be something like
sudo redis-cli -s /var/run/redis/redis.sock flushall
depends where the redis.sock is located.
redis-cli -s /tmp/redis.sock flushall
openvas-scanner’ın start durumu redis.sock dosyasının lokasyonu veya redis-server durumu ile yakından ilgilidir.