I am defining a custom filter like so:
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If by chance this error happens when working with SharePoint 2010: Rename your .json file extensions and be sure to update your restService path. No additional "track by $index" was required.
Luckily I was forwarded this link to this rationale:
.json becomes an important file type in SP2010. SP2010 includes certains webservice endpoints. The location of these files is 14hive\isapi folder. The extension of these files are .json. That is the reason it gives such a error.
"cares only that the contents of a json file is json - not its file extension"
Once the file extensions are changed, should be all set.