Out of nothing, I can no longer get other people's inventory content (using loadUserInventory), even if it's public. The API responds with 403. I can load my own inventory without any issues. (using loadInventory)
After editing the npm package to give more details, i have found out that the API responds with
{"success":false,"Error":"Unsupported request"}
Opening the link in browser (https://steamcommunity.com/profiles/STEAMID/inventory/json/730/2?trading=1) shows the same behaviour. If I put my own ID in the link, I can see all of my items. If I put other person's ID, I get the Unsupported request error. (The person has a public inventory, I can see their items with the normal link - https://steamcommunity.com/profiles/STEAMID/inventory)