In Zabbix 4.2.0 our Zapcat style items are now unsupported. I am not sure if this issue is related to ZBX-13783, since that issue looked like it would fix something like this.

Steps to reproduce:

1. Start to create a new Item under a Host or Template
2. Specify a multi-level (multiple square brackets) Key. eg

   jmx[com.example.app:type=MyWebAp][ErrorResponseCount]

1. Click on Add

Result:
When trying to Add a new Item, we get the current error in the 4.2.0 Frontend: 

We also get the "Invalid key" error message when trying to disable or change the state of the unsupported item.

I believe Zabbix 3.4 still allowed these kind of multi-level (zapcat) keys, and now that we're running Zabbix 4.2.0, the server changed the items to "Not Supported" - see the following issue when hovering over the unsupported item: