We check with SolusVM and received this notice below:
Please be informed that the WHMCS latest version breaks our modules.
You will get the "Unauthorized error" in most of the solusvm button in WHMCS latest version. So you need to upgrade the WHMCS-SOLUSVM module to 3.16.
We have fixed most of the issues with the WHMCS latest version in our recent module with the download located here http://docs.solusvm.com/v2/Default.h...stallation.htm