Whenever you get that alert in Enterprise Manager (this article is based on version 18.104.22.168) and click to see the details, you're left pretty much without useful information - it is not displayed which targets are not uploading data. Now you could click through all homepages of the targets of that host and see when the last upload happened in the All Metrics page, but that gets some serious work when having lots of targets on one machine.
Instead, fill in the host target's name in following SELECT, execute it in the EM repository database and you will see your problem target immediately:
from sysman.mgmt$metric_current mmc
where mmc.target_name=mt.target_name) last_collection
from sysman.mgmt$target mt
where host_name='[name of host target as displayed in EM]'
group by mt.target_name
order by last_collection;
On how to resolve the error - most of the times for databases a reconfiguration already helps (in the target list activate the bullet point next to the database target, click configure - test connection - next - submit). What will most definitely work is what also the Oracle Support will most likely suggest according to my experience: delete and re-add the target. Keep in mind that you will lose the historical data for it in the repository.
Also see following MOS documents: ID 1359328.1, ID 12612391.8