Question :
When the end-user uses the OAuth button to connect to VSA the login fails with Invalid request error or loops back without starting a live connect window.
Answer :
This usually happens if the client was not correctly registered from the VSA. You will have to re-register it. Refer here.
1)In VSA navigate to System>Server Management > OAuth Clients.
2)Register the client by entering your tenant name, Redirect URL (https://<BMS-domain-name>/OAuth/KaseyaVSACallback.aspx) and your email address.
BMS-domain-name is where your tenant is hosted. bms.kaseya.com OR bmsemea.kaseya.com OR www.vorexlogin.com
3)From your email, copy Client ID and Client Secret to BMS Admin > Service Desk > RMM Integration Settings>Liveconnect.
In BMS, Liveconnect icon can be seen in Tickets when Hardware assets are associated with it and in Hardware assets page.
4)Click on the Liveconnect icon in BMS, this should ask you to log in to VSA. If you are already logged in, it should ask you to allow the auth.
Once successfully logged in the icon would no longer show amber in color. 
The next time you click on the live connect button, It should launch the Liveconnect app for that asset.