Data Mining :: MS Excel Data Mining Add-ins / Maximum Number Of Predict Columns

Having successfully created :

- a data mining structure with about 80 columns.
- a data mining model using Microsoft_Decision_Trees with 2 prediction columns. 

I thought I would then explore the possibility of have more than 2 prediction columns, in this case 20.

I get an error message and I can't work out :
a) if this is because there's a limit to the maximum number of prediction columns and where that maximum is stated.
b) if something else has become corrupted
c) there's a know bug and if the error message is either meaningful or not.

Either way, I'm unable to complete the data mining wizard 

The error message is :Errors in the metadata manager. Either the mining structure with the ID of '[my model Structure]' does not exist in the database with the ID of 'DMAddinsDB', or the user does not have permissions to access the object.