So if webhooks are going through for IFTTT it really rules out some sort of firewall or SSL issue.
You likely have some corrupt instance of the smartapp still running in the Smartthings API that is messing things up for you. That is what the cleanup tool mentioned here fixes - Can't Remove Home Assistant smartapp .
The cleanup tool is a python file that runs a script that cleans up the Smartthings API side, so you don’t actually install that file in Hassio. You need to run the python file from another machine that has python installed. I really think you need to run that cleanup file to fix your issue. You download the file to a directory, and then from the directory the file is in, run the command - pip install hass_smartthings_remove . Then you run the command hass_smartthings_remove PERSONAL_ACCESS_TOKEN and replace PERSONAL_ACCESS_TOKEN with your smartthings token. If you lost your old one you can get a new one here - https://account.smartthings.com/tokens . When you make that token, make sure everything is checked to allow the token to access all parts of your Smartthings account.
If you aren’t familiar with python scripts or are running into errors this link should help provide more information on how to do it - https://packaging.python.org/tutorials/installing-packages/ . There’s python 2 and python 3 - python 3 is the newest but I think you might need python 2 to run this script.