Dear Elastic Community
i have elastic agent installed on linux on the host elastic-agent status showing Healthy/running but in fleet server UI it showing always offline ,i did re-installation of the agent but still offline from agent logs i see those errors:
{"log.level":"error","@timestamp":"2026-03-29T09:05:14.914Z","log.origin":{"function":"``github.com/elastic/elastic-agent/internal/pkg/agent/application/gateway/fleet.(*FleetGateway).doExecute","file.name":"fleet/fleet_gateway.go","file.line":220},"message":"Cannot`` checkin in with fleet-server, retrying","log.source":"elastic-agent","error":{"message":"status code: 503, fleet-server returned an error: elastic fail 400: search_phase_execution_exception: all shards failed: illegal_argument_exception: Cannot wait for unissued seqNo checkpoint [wait_for_checkpoint=259, max_issued_seqNo=228], message: elasticsearch error"},"request_duration_ns":3200634848,"failed_checkins":8,"retry_after_ns":830891259763,"ecs.version":"1.6.0"}
{"log.level":"error","@timestamp":"2026-03-29T09:19:06.784Z","log.origin":{"function":"``github.com/elastic/elastic-agent/internal/pkg/agent/application/gateway/fleet.(*FleetGateway).doExecute","file.name":"fleet/fleet_gateway.go","file.line":220},"message":"Cannot`` checkin in with fleet-server, retrying","log.source":"elastic-agent","error":{"message":"status code: 503, fleet-server returned an error: elastic fail 400: search_phase_execution_exception: all shards failed: illegal_argument_exception: Cannot wait for unissued seqNo checkpoint [wait_for_checkpoint=259, max_issued_seqNo=228], message: elasticsearch error"},"request_duration_ns":756109507,"failed_checkins":9,"retry_after_ns":632255999319,"ecs.version":"1.6.0"}
any idea about this issue and how to investigate it
regards