Powerpivot issue - unable to activate and use distinct count

Hello,

I have a data table in a spreadsheet and some pivots driven from this.

I have gone into the file today to have a look at them and when I try to refresh them, or even view the filters, I get the following message:

error.JPG

I have rebuilt the data table in a complete new file, saved it as a .xlsm and then set up a new pivot table but when I click on "add this data to the Data model" and hit OK, I get the same message as above!

I am running the latest office 365 version and have Powerpivot enabled.

I have never seen this before and everything I seem to do to try and fix it is in vain.

Any ideas?

Thanks