Item config is as follows:

key: vserver1
formula: last("lb-live-1:destVserverName.cs_www.example.com_80")
output: Format error or unsupported operator, Exp: ["lbv_wwwNa.example.com_80"]

Key of item being referenced: "destVserverName.cs_www.example.com_80"
Output of the item being referenced: "lbv_www-a.example.com_80"
Type: character

Looks like the calculated item formula doesn't like the format of the output string in the target item.