Connection between SolusVM and WHMCS does not work: Connection error

Symptoms 

• The connection between SolusVM and WHMCS  does not work - the following error is shown on the page WHMCS > a Client > Product/Services > The "Control" filed:
  
• There are missing or outdated ID and Key fields in the SolusVM panel on page SolusVM > Configuration > API Access > Edit API User

Cause

Misconfiguration of WHMCS integration with SolusVM 

Resolution

1. Log in to the SolusVM admin panel
2. Generate a new ID-Key pair by following the guide