For future reference…

When creating credentials (Configuration -> Credentials), this error may appear:

Internal error: create_lsc_credential_omp:4506 (GSA 6.0.1) An internal error occurred while creating a new credential. It is unclear whether the credential has been created or not. Diagnostics: Failure to receive response from manager daemon.

The solution is to run these commands:

openvasmd --create-credentials-encryption-key
mkdir /var/lib/openvas/gnupg