We have been running into an issue where the check now option on the item page returns the following error

To replicate this,
goto the items page on any host.
in the filter section, remove all selections from all fields except the item name.
add the name for an item to the name field in the filter section that is used on several hosts. (agent.hostname as an example)
click apply.
ensure that one of the results that was returned belongs to a host that is disabled.
select (check) all the returned results and select the "check now" option.