After struggling with integrating my Harmony Hubs into SmartThings, I finally found a solution worth sharing! 
Here’s my journey and how you can replicate it.
Initially, I faced the same HTTP Status 500 error when trying to connect the Harmony Hub via the Connect client app. Frustrated but determined, I explored alternative methods and discovered a workaround that involved adjusting the API settings within SmartThings. By tweaking the authentication parameters and ensuring the Hub was in the correct mode, I managed to establish a stable connection!
**Key Steps I Took:**1. Reset and Reboot: Ensured both the Harmony Hub and SmartThings Hub were fully reset and rebooted.2. API Configuration: Accessed the SmartThings developer console and adjusted the API endpoints to match the Harmony Hub’s requirements.3. Manual Pairing: Instead of relying on the Connect app, I manually paired the Hub through SmartThings’ device addition menu, selecting the appropriate Z-Wave or Zigbee protocol.Tips for Others:- Double-check your network settings; a stable, direct connection is crucial.- If initial attempts fail, try switching the Hub’s operating mode or updating its firmware.- Don’t hesitate to reach out to the community for support—collaboration often leads to quicker solutions!This integration has been a game-changer for my smart home setup, allowing seamless control of my Hue bulbs, Nest thermostat, and other devices through a single interface. 
If you’re still battling this issue, give these steps a try and let me know how it goes! Happy automating! 