OpenShift Data Foundation(ODF), it uses the OCP Service CA. By default, the lokistack doesn't trust the OCP Service CA. It is better to provide a step to trust the Service CA when the storage type is ODF.

Suggest adding https://docs.openshift.com/container-platform/4.15/observability/logging/log_storage/installing-log-storage.html#logging-loki-storage-odf_installing-log-storage.

step 4: Enable Object Storage ODF in lokistack.

cat << EOF | oc apply -f -
apiVersion: loki.grafana.com/v1
kind: LokiStack
metadata:
  name: lokistack-sample
  namespace: openshift-logging
spec:
  managementState: Managed
  size: 1x.extra-small
  storage:
    secret:
      name: s3-secret
      type: s3
    tls:
      caName: openshift-service-ca.crt
  storageClassName: gp2
  tenants:
    mode: openshift-logging
EOF

Note: If the ODF is not in same cluster or you are using ODF route URL, you can create a configmap and bind to the lokistack.
oc create configmap lokistack-odf-ca --from-file=service-ca.crt=odf.crt