There is a max length to each of the fields being passed through to Dimensions. If the length of the data in a field is too long, it can cause this error. It may be a description or reference field that is triggering this error.
You can use the Excel function LEN (e.g. =LEN(Cell Reference) to determine how many characters are in a field.
Below are the maximum lengths of fields for a journal:
Journal Ref (Max Length 10)
Description (Max Length 200)
Batch Ref (Max Length 10)
Detail (Max Length 240)
Project Code (Max Length 10)
CostCentre (Max Length 10)
Transactions key (Max Length 20)