It is always a good idea when dealing with IM to do some trouble shooting by attempting to enter the data manually. There are sometimes things like a dialog box that pops up which was not anticipated by the designers of IM. In this case it appears that something is wrong with the document numbers being imported.
One more thought on this. Just as an example. If you had the default doc sequence looking like INV999. If you tried to feed it a document number of INV1000 that would through the error you got. If that is the case, change the sequence to INV001000. That would take you to INV99999 before you got this error again.
Warren