Usually this indicates a technical error occurred during the time that the invoice was retrieved by the client's system. The client's system did not know how to process the invoice and thus it was rejected. It is difficult to find information from the Collaborati side to indicate the reason behind the error.
Try the following :
1) Contact your client and let them know that your invoice was rejected due to a technical error. It might be helpful to send the client a pdf of the rejected invoice from Collaborati showing the comments and details.
2) Resubmit the invoice. If the invoice is rejected again (for the same reason), the client should investigate by checking the logs corresponding to the time of the invoice rejection - this can be obtained by reviewing the Invoice History page in Collaborati.