Symptoms
Error during registration wizard, step "SharePoint Service Broker":
VALIDATION The remote server returned an error: (403) Forbidden. Failed to initialize the Context: URL: http://Domain/Environment Username: Error Details: Method SharePointService.initializeContext x-ms-diagnostics: 30000000 reason = SSL is required SPRequestGuid: xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx
Diagnoses
The issue could appears if SharePoint is in HTTPS and K2 is in HTTP (or reverse).
Resolution
This issue was resolved by running the following Powershell script on the customers SharePoint server:
$stsc = Get-SPSecurityTokenServiceConfig
$stsc.AllowMetadataOverHttp = $true
$stsc.AllowOAuthOverHttp= $true
$stsc.Update()
This allows the ability to use OAuth over http.