Users are able to select non operational Configuration Items(CIs) at times while creating task records. But while trying to select the using the reference popup, these will not be shown. There are server side scripts in place to hide these from selection.
All available releases.
The templates these users have might be setting these values. If you check the node logs when these task records are created, you will see that they are using the sys_templates which has non-operational CIs selected for the Configuration Items field. The log will look something like the following.
2019-05-07 12:22:55 (911) Default-thread-7SESSION_ID txid=TRANSACTION_ID #11700576 /xmlhttp.do Parameters -------------------------
Remove the non-operational CI from the sys_template record and that will resolve the issue.