It is true what Brian indicates, that is the hotfix from WHMCS: CORE-10723
I cannot confirm that it will help as we have removed the empty function, before the fix came out, which already solved it for us.
For that solution you'll have to find the php file, located here: /modules/servers/<solusvm dir>/<solusvm php file>
In that file locate the function <solusvm>_AdminCustomButtonArray(), if that funtion returns an empty array it's best to remove the whole function (comment it out).
But this only works if the file is not encoded and since most third-party modules are, the only option is the WHMCS fix.
It should be already insite the latest WHMCS version, so an upgrade there should also solve it ;-)
Good luck!