A checksum error means there was a problem with the serial communications. The checksum is at it's simplest the sum of all of the bytes from the the start of a data string to the end. This is generally encoded at the end of the data string and used to check for communication errors. A checksum error occurs when the computed checksum on the receiving end doesn't match the transmitted value. Generally, this means there's some noise on the communications line and one of the bits was mis-transmitted or something similar, although in this case it could be due to the data buffer overflow and the data strings getting a little out of sync. For example, a data string could start and then the buffer overflow error occurs, so a portion of that data string is lost. The Vectrino software picks up the stream from the buffer but is missing a little bit so the checksums don't match.
The checksum error typically appears when a cable is damaged or a recorder failure. We recommend trying with another cable and performing a continuity test on the cable that is suspect. It can also be worth updating the software version if an old one is currently used.
For an understanding of how the instrument calculates the checksum, check out the relevant System Integrator Manual.