Prometheus alerts are failing after grafana upgrade

I installed and configured successfully prometheus (in openshift) data source in grafana, and set some panels and alerts for more than 1 year.
After a grafana upgrade from 8.1.4 to 8.3.4, the graphs and panels are still working, but the alerts are now failing with error:
t=2022-02-07T15:05:07+0000 lvl=eror msg=“Range query failed” logger=tsdb.prometheus query=“instance:node_cpu_utilisation:rate1m{job=“node-exporter”, instance=“the-openshift-node”}” err=“client_error: client error: 403”
t=2022-02-07T15:05:07+0000 lvl=eror msg=“Alert Rule Result Error” logger=alerting.evalContext ruleId=223 name=“PROD OCP Nodes - CPU alert” error=“request handler response error {client_error: client error: 403 A 0xc0013d63f0}” changing state to=alerting

Can you help, please?
Waht can be wrong?

Anybody can give me some hints?