OK you have a somewhat strange problem indeed! Never come accross this one.
I find it very curious that your IPN history is empty. I assume that by “removed all data” you mean that you also emptied the wp_wallets_adds table. As I mentioned before, this data is not deleted when you uninstall the plugin.
Here’s what you can check:
Every time the plugin requests a new deposit address, a “callback address” is generated on the CoinPayments platform. You can then view these callback addresses via the dashboard. For Litecoin testnet, navigate to https://www.coinpayments.net/index.php?cmd=acct_balances&action=deposit&coin=LTCT
You should see your deposit address listed there. It’s type should be “commercial”. If that’s the case, then when you send funds to that address the platform should attempt an IPN to your site, regardless of what the plugin, or anything on your server, does.