It seems like there was a regression in JSON Path parsing due to the changes in 4.0.11 (https://www.zabbix.com/rn/rn4.0.11), I built a template on 4.0.10 which worked fine until I migrated to 4.0.16.

Steps to reproduce:

1. create master item with JSON data, i.e.

   {"D:\\Xyz": {"free": 1234}}

2. create discovery that returns

   {#NAME} = "D:\Xyz"

   (note single backslash)

3. create dependent item inside discovery
4. add preprocessing

   JSON Path -> $['{#NAME}'].free

5. link to hosts, wait for discovery and keys
6. item preprocessing fails

So, simply put, you can't use single backslashes in the JSON Path parameter anymore, they need to be escaped?

I know why this happens but I think the correct behavior is to allow single backslashes in user-facing fields (except maybe regex fields) and fix them in the backend if necessary.

jsonpath.com seems to accept single backslashes (attachment)...