Zoho Inventory Item Creation API

We have been using Zoho Inventory API for several years to create items in the portal successfully. Suddenly we get the common and uninformative error response {'code': 4, 'message': 'Invalid value passed for JSONString'}. I have checked with our Zoho Dev and he assures me that nothing has changed as far as our customised configuration for items. I have tried to create even the most basic item with just a name and sku without any luck.

As with most issues with zoho no information is given about the specific field in question. In the past this usually means that there is a required field that isn't being set that needs to be. The API docs are rarely updated so I don't know if I can trust that the only required field is name.

I need help to diagnose what in the below configuration is misconfigured or not filled out properly. Some of these fields are dropdowns. Like I mentioned earlier we have been using this for years and those values definitely exist in the dropdowns. The customfield id is also correct and corresponds to the the correct field with that dropdown value.

Has there been a change to the required fields for item creation? Is there a change to how we create a record with a custom field. Could this error message be misleading and actually be related to something else? I have ensured that we are able to read and write for other objects using our auth so that's not the issue.

Thanks in advance

{
   "sku":"PCK02057",
   "name":"CA_PRINTER_LABELS_4X2_SECONDARY_PACK",
   "unit":"Unit(s)",
   "purchase_account_name":"Cost of Goods Sold",
   "inventory_account_name":"Inventory Asset",
   "item_type":"inventory",
   "account_name":"Sales",
   "purchase_description":"PCK02057",
   "description":"PCK02057",
   "custom_fields":[
      {
         "customfield_id":"109027000000032124",
         "value":"Packaging"
      }
   ]
}