This is most likely caused by some outdated SSL/TLS certificates for the communication between Scanner (openvassd) and Manager (openvasmd).
It seems you’re running a quite outdated and end-of-life version of the Manager (See GVM-9 (stable, initial release 2017-03-07) for the current and supported versions).
For now i would suggest that you upgrade to the recent GVM-9 release. In this release the communication between the openvassd and openvasmd is done via file/unix sockets where you don’t need to care about SSL/TLS certificates anymore.