Updating from Excel - "External Table not in the expected format"; what 2 check?

[miless2111s]

I have a piece of excel code that generates a fairly large data set in a named range in an Excel sheet.
I have around 1.5K shapes in Visio linked to this data set which are updated using the "Update database" function in Visio.

This has worked OK on the work's machine for some time however last month it stopped working displaying the error message:  "External table is not in the expected format".  When attempting to refresh only one shape (to avoid being a 1.5K times loop) I also get an error message at the end of the failed process of "Can not connect to the data source 'excel files'.  Run the wizard and re-link the shape to correct the problem".

I have developed the code on this machine and another machine so there is the possibility that they are or 32 bit and 64 bit??

As a side not it is possible to connect to another, older (manually produced) data range which successfully updates however it is horribly slow.

1) Is anyone out there familiar with this error?
2) It is possible that my excel code has done something odd but what should I be checking for in terms of format or layout?
3) What can I check / change on the machine to make it work

Many thanks

Miles