How do you resolve this error message -- error processing upload document: null

We are using web ADI to upload expenditure items to Projects.  We have been doing it for a while now, and it has been successful as recently as last week.   Today when we run it, it errors but the message is only    error processing upload document:  null        The data is very similar to last week, same projects, tasks, expenditure types, organizations, similar quantity/cost, date is one week later but still in the middle of the March period.

Have you seen the null message before?  If so, how did you resolve it?

Where might I find a list of all web ADI error messages, to see if   null    is a known problem?