Not sure about B1, but if this was in the full SAP ECC 6.0, I'd suggest that it might be due to a faulty record in the relevant table.
For example, if it is the "Customer number" field causing the issue, then there might be an entry that doesn't match the correct format. This can be caused by a number of issues; if you can find the relevant table (OCRD) you should be able to look through to see if anything appears out of the ordinary.
