stracing the error leads to this :
[pid 16787] open("/var/lib/knot/external/keys/policy_\\x06policy.json",
O_RDONLY) = -1 ENOENT (No such file or directory)
Hm. This looks like a clash between KASP database and dnssec-policy
config option.
That \x06 probably means that the policy name was used in the domain
name wire format where ASCII name was expected. Daniel?
Jan