I have checked the log and try putting required field one by one and came to know this was happening due to Severity field of Incident, though this field is coming from OVO message and getting populated also in the service desk severity. But once I make it mandatory i get the problem in the log.
The out-of-the-box import mapping takes the OVOU Severity value and maps it to the SD impact field. There is no severity field in SD by default. You will have to show us your mapping if you have created a field called Severity in SD.