I copied and pasted your code and I got the same message. I then changed the filename parameter to be an absolute path and it worked. Apparently relative paths don't work with the saveas method.
Just incase somebody else was interested here is the code to get the drive letter.
set myString to type Environment::CurrentDirectory()
set myString to type Path::GetPathRoot(myString)
set myString to myString & "temp\test1.xlsx"
I will create a support incident for this asap.