K2 Five App Registration SharePoint Online

Hi Guys,

We have an existing K2 Five (5.0) environment On Prem working with our On Prem SharePoint 2016. We’ve now migrated our Exchange and SharePoint operations to Office 365 (SharePoint Online). I have created a new App Catalogue in SharePoint Online and added the K2 Five app successfully from the Microsoft App Store. 

Now, when I try to Register K2 Five the following happens: 
 

1. I enter our K2 server address - it is already publicly accessible and behind a https://
2. It shows ‘verifying relaying’
3. Asks me to login to Office 365 - I use the O365 Admin account
4. I get a Microsoft Prompt to Allow K2 for Office 365 a bunch of permissions
5. Trust.k2.com comes up verifying permissions and then automatically goes to
6. App Only configured for Tenant with a bunch of permissions automatically
7. and then it tries to go back to the App address of the K2 app and I get a 500 Internal Server Error

Not sure what to do to get this to register properly - did I miss something?

Any ideas and suggestions appreciated ;)